drpc compatibility with regards to JSONMarshal and JSONUnmarshal.

[egonelbre]

drpc expects JSONMarshal and JSONUnmarshal to be available. This mostly should be equivalent to just calling json.Marshal and json.Unmarshal.

func (drpcEncoding_File_certificate_proto) JSONMarshal(msg drpc.Message) ([]byte, error) {
	return picobuf.JSONMarshal(msg)
}

func (drpcEncoding_File_certificate_proto) JSONUnmarshal(buf []byte, msg drpc.Message) error {
	return picobuf.JSONUnmarshal(buf, msg)
}

However, we may need to ensure and document that custom types may need a custom encoding as well. Either way, we need to verify that things work in a gogo-protobuf compatible way.