Generics missing for `PackedScene::instance`

I have a `Mob` class and a `PackedScene` with the root node of type `Mob`. I would like to instance the scene like this:

```cpp
Mob mob = mob_scene->instance();
```

However, there are no generics for `PackedScene::instance`. These exist in C# in Godot 3.3 and later, but not GDNative C++. The solution would be to add this feature to the GDNative C++ bindings.