[Enhansment] Expose more functions MultiplayerSpawner funtions [instantiate_custom,instantiate_scene] to godot-cpp?

For more flexible multiplayer functionality more `MultiplayerSpawner` might be provided for overriding.

```
Node *spawn(const Variant &p_data = Variant());
Node *instantiate_custom(const Variant &p_data);
Node *instantiate_scene(int p_idx);
```

also access to `HashMap tracked_nodes;` would be very helpfull.