[#405] Convert object to/from JSON

[jabolina]

The field IDs for Instant and Date were ignored during the conversion so the generated JSON was an empty string. The fix was a bit more convoluted. I'll explain all I have and maybe there's a better solution.

• An Instant is converted by the WrappedMessage to byte[] by writing one int64 field for seconds and int32 for nanos.
• The Proto to JSON conversion needs to read two fields now. The message-wrapping.proto file describes the seconds part as a oneOf which writes the key as "_value". We still need to write to fields, so we end with {"_value": 0, "wrappedInstantNanos": 0} without any typing.
• We could use the type as a WrappedMessage, but the conversion from JSON to Instant would fail as it creates the Instant inside the WrappedMessage object. So we need an explicit type for the conversion to create the correct object.

Since we need this conversion from JSON to Instant to work, I've created an adapter in the types module. During the Proto to JSON conversion, we require this adapter to be registered so we can write the field names and the type correctly.

We are still left with converting JSON to Proto byte[]. We need to verify in the WrappedMessage class whether this object corresponds to a known type. Otherwise, the generated byte[] will be entirely different from the one produced by Instant to Proto byte[] serialization.

The Date object suffers from the same thing. The difference is that:

• We serialize Date as an int64 from the Date#getTime method.
• This would create a JSON as {"_type": "int64", "_value": 12345}. Which loses any type parameter that this value is a Date object.
• The conversion from JSON to Date will fail as it returns a primitive long number instead of the Date object.

Closes #405.