Add a instance callback for extensions

This sends the Godot object instance back to the extension so they can
keep a pointer for function calls.

Incidentally fix argument order on instance bindings callback for free()

core/object/object.h

@@ -272,6 +272,7 @@ struct ObjectNativeExtension {
 
 	GDNativeExtensionClassCreateInstance create_instance;
 	GDNativeExtensionClassFreeInstance free_instance;
+	GDNativeExtensionClassObjectInstance set_object_instance;
 	GDNativeExtensionClassGetVirtual get_virtual;
 };