Support Protobuf JSON

[Dogacel]

Make sure well-known-types are also handled correctly. Full details here

For example JSON encoding for a Timestamp is not { "seconds": 10, "nanos": 0 } but rather an ISO string such as "2020-01-01T12:00:00Z".

Put the necessary annotations to the generated code.

Supporting both snake case and json_name option is optional for this issue.