Remove player entity context from player offers

[Gegy]

Smaller refactors than hoped, but better to get part of the work out of the way anyway rather than be stuck forever. 😅
With the goal to split games across multiple server instances, we want to be able to request that a player joins without having the player physically on the server yet. Most games probably do not care about working with a ServerPlayerEntity, so just passing the GameProfile should be enough even for the most exotic cases (e.g. player rejoining a game).