Support `onMessage` parsing `Schema` messages sent from client to server

The work @endel did for issue https://github.com/colyseus/colyseus/issues/315 is extremely useful for sharing Schemas between the client and server for messages that the server sends to the client, and it would be great to have that same functionality available when sending messages from the client to the server.

Currently, everyone needs to manually implement parsing JSON messages from the client, which is error-prone and a hassle. Additionally, implementing this would let us benefit from using schema-codegen to keep message schemas the same between the client and server.

It could look something like this, similar to what’s listed in https://github.com/colyseus/colyseus/issues/315:

this.onMessage(MoveMessage, (client: Client, message: MoveMessage) => {
  // Handle schema-encoded MoveMessage, while benefitting from autocompletion.
});

[endel]

Hi @rnd256, thanks! I agree, let’s use this issue to track the progress on the documentation https://github.com/colyseus/colyseus/issues/433

[rnd256]

I think this is still relevant - ideally the docs would cover this helpful feature