Canonical protobuf json format support

[vietj]

The json protobuf format is optimised for vertx JSON types (e.g it can a an integer type) to get more compact json representations. We should also support this format https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/struct.proto and be wire compatible with it (which means using the same tags), so our messages can be compatible with com.google.protobuf.Struct

That would be declared on the @Protobuf annotation, e.g @Protobuf(compactJson=false)

[lwlee2608]

I will be working on this.