ProtoJSONPayload support

[stuppy]

Go impl: https://github.com/temporalio/sdk-go/blob/master/converter/proto_json_payload_converter.go

Today, if a proto Message is passed as a payload, the JSON to_payload Temporal::JSON.serialize(data) into what looks like the proto class descriptor and not the data within the message.

[antstorm]

This issue can probably be closed now

[stuppy]

#192 indeed! 🙇