[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.

scene/SCsub

@@ -9,7 +9,6 @@ env.add_source_files(env.scene_sources, "*.cpp")
 
 # Chain load SCsubs
 SConscript("main/SCsub")
-SConscript("multiplayer/SCsub")
 SConscript("gui/SCsub")
 if not env["disable_3d"]:
     SConscript("3d/SCsub")