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

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.

Pull-Request: #2302
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>