[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/gdscript/gdscript_compiler.cpp

@@ -1975,7 +1975,7 @@ GDScriptFunction *GDScriptCompiler::_parse_function(Error &r_error, GDScript *p_
 
 	StringName func_name;
 	bool is_static = false;
-	Multiplayer::RPCConfig rpc_config;
+	Variant rpc_config;
 	GDScriptDataType return_type;
 	return_type.has_type = true;
 	return_type.kind = GDScriptDataType::BUILTIN;