sidekick: generated code uses ProtoJSON for serialization / deserialization

As we all know, ProtoJSON != JSON. We need to design an approach to serialize and deserialize the generated types to and from ProtoJSON.

This maybe in the form of a new set of protocols they implement (not `Encodable`). Or maybe they use custom encoders and decoders.

Part of the work will be in the generator, to use these protocols or custom encoders, part of the work will be in gax.