signature: Add CapabilityTeller interface

This new interface, currently with a single method that indicates
whether the event can be signed, is used to return an error if
the event given to Signer.Sign is too old to support signing.
All events implement the CapabilityTeller interface. In the future
we might want to add additional methods for expressing other
capabilities of events, e.g. if they support meta.source.domainId.

EiffelActivityCanceledEventV1.go

@@ -89,8 +89,11 @@ func (e *ActivityCanceledV1) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityCanceledV1{}
-var _ MetaTeller = &ActivityCanceledV1{}
+var (
+	_ CapabilityTeller = &ActivityCanceledV1{}
+	_ FieldSetter      = &ActivityCanceledV1{}
+	_ MetaTeller       = &ActivityCanceledV1{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityCanceledV1) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityCanceledV1) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityCanceledV1) SupportsSigning() bool {
+	return false
+}
+
 type ActivityCanceledV1 struct {
 	// Mandatory fields
 	Data  ActCV1Data   `json:"data"`

EiffelActivityCanceledEventV2.go

@@ -89,8 +89,11 @@ func (e *ActivityCanceledV2) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityCanceledV2{}
-var _ MetaTeller = &ActivityCanceledV2{}
+var (
+	_ CapabilityTeller = &ActivityCanceledV2{}
+	_ FieldSetter      = &ActivityCanceledV2{}
+	_ MetaTeller       = &ActivityCanceledV2{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityCanceledV2) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityCanceledV2) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityCanceledV2) SupportsSigning() bool {
+	return false
+}
+
 type ActivityCanceledV2 struct {
 	// Mandatory fields
 	Data  ActCV2Data   `json:"data"`

EiffelActivityCanceledEventV3.go

@@ -89,8 +89,11 @@ func (e *ActivityCanceledV3) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityCanceledV3{}
-var _ MetaTeller = &ActivityCanceledV3{}
+var (
+	_ CapabilityTeller = &ActivityCanceledV3{}
+	_ FieldSetter      = &ActivityCanceledV3{}
+	_ MetaTeller       = &ActivityCanceledV3{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityCanceledV3) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityCanceledV3) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityCanceledV3) SupportsSigning() bool {
+	return true
+}
+
 type ActivityCanceledV3 struct {
 	// Mandatory fields
 	Data  ActCV3Data   `json:"data"`

EiffelActivityFinishedEventV1.go

@@ -89,8 +89,11 @@ func (e *ActivityFinishedV1) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityFinishedV1{}
-var _ MetaTeller = &ActivityFinishedV1{}
+var (
+	_ CapabilityTeller = &ActivityFinishedV1{}
+	_ FieldSetter      = &ActivityFinishedV1{}
+	_ MetaTeller       = &ActivityFinishedV1{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityFinishedV1) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityFinishedV1) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityFinishedV1) SupportsSigning() bool {
+	return false
+}
+
 type ActivityFinishedV1 struct {
 	// Mandatory fields
 	Data  ActFV1Data   `json:"data"`

EiffelActivityFinishedEventV2.go

@@ -89,8 +89,11 @@ func (e *ActivityFinishedV2) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityFinishedV2{}
-var _ MetaTeller = &ActivityFinishedV2{}
+var (
+	_ CapabilityTeller = &ActivityFinishedV2{}
+	_ FieldSetter      = &ActivityFinishedV2{}
+	_ MetaTeller       = &ActivityFinishedV2{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityFinishedV2) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityFinishedV2) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityFinishedV2) SupportsSigning() bool {
+	return false
+}
+
 type ActivityFinishedV2 struct {
 	// Mandatory fields
 	Data  ActFV2Data   `json:"data"`

EiffelActivityFinishedEventV3.go

@@ -89,8 +89,11 @@ func (e *ActivityFinishedV3) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityFinishedV3{}
-var _ MetaTeller = &ActivityFinishedV3{}
+var (
+	_ CapabilityTeller = &ActivityFinishedV3{}
+	_ FieldSetter      = &ActivityFinishedV3{}
+	_ MetaTeller       = &ActivityFinishedV3{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityFinishedV3) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityFinishedV3) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityFinishedV3) SupportsSigning() bool {
+	return true
+}
+
 type ActivityFinishedV3 struct {
 	// Mandatory fields
 	Data  ActFV3Data   `json:"data"`

EiffelActivityStartedEventV1.go

@@ -89,8 +89,11 @@ func (e *ActivityStartedV1) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityStartedV1{}
-var _ MetaTeller = &ActivityStartedV1{}
+var (
+	_ CapabilityTeller = &ActivityStartedV1{}
+	_ FieldSetter      = &ActivityStartedV1{}
+	_ MetaTeller       = &ActivityStartedV1{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityStartedV1) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityStartedV1) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityStartedV1) SupportsSigning() bool {
+	return false
+}
+
 type ActivityStartedV1 struct {
 	// Mandatory fields
 	Data  ActSV1Data   `json:"data"`

EiffelActivityStartedEventV2.go

@@ -89,8 +89,11 @@ func (e *ActivityStartedV2) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityStartedV2{}
-var _ MetaTeller = &ActivityStartedV2{}
+var (
+	_ CapabilityTeller = &ActivityStartedV2{}
+	_ FieldSetter      = &ActivityStartedV2{}
+	_ MetaTeller       = &ActivityStartedV2{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityStartedV2) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityStartedV2) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityStartedV2) SupportsSigning() bool {
+	return false
+}
+
 type ActivityStartedV2 struct {
 	// Mandatory fields
 	Data  ActSV2Data   `json:"data"`

EiffelActivityStartedEventV3.go

@@ -89,8 +89,11 @@ func (e *ActivityStartedV3) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityStartedV3{}
-var _ MetaTeller = &ActivityStartedV3{}
+var (
+	_ CapabilityTeller = &ActivityStartedV3{}
+	_ FieldSetter      = &ActivityStartedV3{}
+	_ MetaTeller       = &ActivityStartedV3{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityStartedV3) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityStartedV3) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityStartedV3) SupportsSigning() bool {
+	return false
+}
+
 type ActivityStartedV3 struct {
 	// Mandatory fields
 	Data  ActSV3Data   `json:"data"`

EiffelActivityStartedEventV4.go

@@ -89,8 +89,11 @@ func (e *ActivityStartedV4) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityStartedV4{}
-var _ MetaTeller = &ActivityStartedV4{}
+var (
+	_ CapabilityTeller = &ActivityStartedV4{}
+	_ FieldSetter      = &ActivityStartedV4{}
+	_ MetaTeller       = &ActivityStartedV4{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityStartedV4) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityStartedV4) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityStartedV4) SupportsSigning() bool {
+	return true
+}
+
 type ActivityStartedV4 struct {
 	// Mandatory fields
 	Data  ActSV4Data   `json:"data"`

EiffelActivityTriggeredEventV1.go

@@ -89,8 +89,11 @@ func (e *ActivityTriggeredV1) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityTriggeredV1{}
-var _ MetaTeller = &ActivityTriggeredV1{}
+var (
+	_ CapabilityTeller = &ActivityTriggeredV1{}
+	_ FieldSetter      = &ActivityTriggeredV1{}
+	_ MetaTeller       = &ActivityTriggeredV1{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityTriggeredV1) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityTriggeredV1) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityTriggeredV1) SupportsSigning() bool {
+	return false
+}
+
 type ActivityTriggeredV1 struct {
 	// Mandatory fields
 	Data  ActTV1Data   `json:"data"`

EiffelActivityTriggeredEventV2.go

@@ -89,8 +89,11 @@ func (e *ActivityTriggeredV2) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityTriggeredV2{}
-var _ MetaTeller = &ActivityTriggeredV2{}
+var (
+	_ CapabilityTeller = &ActivityTriggeredV2{}
+	_ FieldSetter      = &ActivityTriggeredV2{}
+	_ MetaTeller       = &ActivityTriggeredV2{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityTriggeredV2) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityTriggeredV2) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityTriggeredV2) SupportsSigning() bool {
+	return false
+}
+
 type ActivityTriggeredV2 struct {
 	// Mandatory fields
 	Data  ActTV2Data   `json:"data"`

EiffelActivityTriggeredEventV3.go

@@ -89,8 +89,11 @@ func (e *ActivityTriggeredV3) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityTriggeredV3{}
-var _ MetaTeller = &ActivityTriggeredV3{}
+var (
+	_ CapabilityTeller = &ActivityTriggeredV3{}
+	_ FieldSetter      = &ActivityTriggeredV3{}
+	_ MetaTeller       = &ActivityTriggeredV3{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityTriggeredV3) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityTriggeredV3) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityTriggeredV3) SupportsSigning() bool {
+	return false
+}
+
 type ActivityTriggeredV3 struct {
 	// Mandatory fields
 	Data  ActTV3Data   `json:"data"`

EiffelActivityTriggeredEventV4.go

@@ -89,8 +89,11 @@ func (e *ActivityTriggeredV4) String() string {
 	return string(b)
 }
 
-var _ FieldSetter = &ActivityTriggeredV4{}
-var _ MetaTeller = &ActivityTriggeredV4{}
+var (
+	_ CapabilityTeller = &ActivityTriggeredV4{}
+	_ FieldSetter      = &ActivityTriggeredV4{}
+	_ MetaTeller       = &ActivityTriggeredV4{}
+)
 
 // ID returns the value of the meta.id field.
 func (e ActivityTriggeredV4) ID() string {
@@ -117,6 +120,13 @@ func (e ActivityTriggeredV4) DomainID() string {
 	return e.Meta.Source.DomainID
 }
 
+// SupportsSigning returns true if the event supports signatures according
+// to V3 of the meta field, i.e. events where the signature is found under
+// meta.security.integrityProtection.
+func (e ActivityTriggeredV4) SupportsSigning() bool {
+	return true
+}
+
 type ActivityTriggeredV4 struct {
 	// Mandatory fields
 	Data  ActTV4Data   `json:"data"`