[Feature] Add validation for Definition Object

[wonderflow]

For traitDefinition, the value of applyTo could have many formats:

• format by group/version.Kind => core.oam.dev/v1alpha2.PodspecWorkload
• directly use workloadDefinition Name => webservice
• directly use CRD name => deployments.apps
• use group name to represent apply to the whole group => core.oam.dev

We need to add validation for it and future needs.

[LeoLiuYan]

Now, when apply traits, the workloads does not add to traitDefinition's AppliesToWorkloads field like the scopes apply do. So, the applyTo field maybe not use in this moment.

The associated code maybe here:
https://github.com/crossplane/oam-kubernetes-runtime/blob/master/pkg/controller/v1alpha2/applicationconfiguration/apply.go#L104
https://github.com/crossplane/oam-kubernetes-runtime/blob/master/pkg/controller/v1alpha2/applicationconfiguration/apply.go#L121

[wonderflow]

Yes, checking applyTo fields should also add to this validation. @LeoLiuYan

[LeoLiuYan]

Could this feature assign to me? @wonderflow

[wonderflow]

Sure, we have already setup validation webhook framework https://github.com/crossplane/oam-kubernetes-runtime/tree/master/pkg/webhook/v1alpha2

You could add it there. @LeoLiuYan