Merge KaraokeEvent.UsernameSet and the client-to-server KaraokeEvent.RoomDataHydration

[JoshuaKGoldberg]

Following #9: there are two initial messages sent from a client to the server when it joins a room:

1. KaraokeEvent.RoomDataHydration: A request for the basic room data, made in DynamicSceneHydrating -> useRoomDataConnection.
2. KaraokeEvent.UsernameSet: Indicates to the server what the client username is. No response is waited for.

For the sake of simplicity & fewer messages, we should really join these two into a single one, named something like KaraokeNite.RoomJoin. The client can then send the username in step 1 and still hydrate room data in that same step -- instead of waiting until step 2 to set username.