json::serialization: don't raise errors on missing attributes with default values

[xymus]

This PR fits in the ongoing work to improve deserializing from plain JSON objects.

The JSON deserialization engine no longer raises errors on missing attributes when a default value is available. Attributes may be missing because the JSON object come from a third-party API or
when loading serialized data from a previous version of the software. Default values include simple default values (var x = 4), lazy attributes and nullable types (which are set to null). This does not yet include optional attributes, more work would be needed.

The test/example can be activated when #2296 is fixed.

[privat]

nice
+1

[Morriar]

Cool, +1

[ppepos]

Is there a possibility to add something similar to how errors are managed to know if an attribute was missing (and has been set to the default value)? I feel like being capable of knowing if the object was exactly as expected has its benefits.

[xymus]

@ppepos There could be a Deserializer::warnings alongside errors for such cases. It could be useful at debugging or when loading data from a previous software version to detect possible errors and warn the user. This could easily be added if there's a need for it.

Another alternative would be an annotation to change the behavior per attribute and make then non-optional. (serialize_no_default? serialize@force?) This would apply to classes where the default values are really constructors.

[ppepos]

Option 1 seems to cover my worries.