[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.

tests/core/object/test_object.h

@@ -95,8 +95,8 @@ public:
 	Ref<Script> get_script() const override {
 		return Ref<Script>();
 	}
-	const Vector<Multiplayer::RPCConfig> get_rpc_methods() const override {
-		return Vector<Multiplayer::RPCConfig>();
+	const Variant get_rpc_config() const override {
+		return Variant();
 	}
 	ScriptLanguage *get_language() override {
 		return nullptr;