Change proposal: add intent field to messages

[emergent-reasons]

This came from @alwaysAn0n . Currently clients need to infer quite a lot about each message and it seems to add unnecessary complexity to the client.

As long as the shuffle procedure is narrow and well-characterized, would it make sense to signal the intent of each message?

For example the registration message has a clear format and intent that makes it easy for the server to identify and work with explicitly.

The counter argument is that a non-generic structure will be harder to extend. However if there is just an intent field, that could be extended for different message types. In other words, each message that a client needs to send could have a specific enum attached to it. If the protocol changes or grows, old message types can be deprecated and new ones added.

Comments from someone with experience extending and growing APIs/protobuf protocols would be helpful here.