[Net] Modularize multiplayer, expose MultiplayerAPI to extensions.

- RPC configurations are now dictionaries.
- Script.get_rpc_methods renamed to Script.get_rpc_config.
- Node.rpc[_id] and Callable.rpc now return an Error.
- Refactor MultiplayerAPI to allow extension.
- New MultiplayerAPI.rpc method with Array argument (for scripts).
- Move the default MultiplayerAPI implementation to a module.

modules/websocket/websocket_multiplayer_peer.h

@@ -32,8 +32,8 @@
 #define WEBSOCKET_MULTIPLAYER_PEER_H
 
 #include "core/error/error_list.h"
-#include "core/multiplayer/multiplayer_peer.h"
 #include "core/templates/list.h"
+#include "scene/main/multiplayer_peer.h"
 #include "websocket_peer.h"
 
 class WebSocketMultiplayerPeer : public MultiplayerPeer {