Move multiplayer classes to "core/multiplayer" subdir.
Move the RPCConfig and enums (TransferMode, RPCMode) to a separate
file (multiplayer.h), and bind them to the global namespace.
Move the RPC handling code to its own class (RPCManager).
Renames "get_rpc_sender_id" to "get_remote_sender_id".
Used by ENetMultiplayerPeer and WebSocketServer to generate network IDs,
and exposed to the user for p2p networks (e.g. WebRTCMultiplayerPeer)
and custom MultiplayerPeer implementations.
From empirical testing, this seems to provide the best compression
compared to other compression algorithms when used in the
Multiplayer Bomber demo.
Other algorithms may provide better compression ratios for more
complex games, but some compression is probably better than
no compression.
Zstandard was also not very efficient in my testing, so I added
a note in the documentation.
