Change gdnative interface so that Godot object initialization is done from the extension side.

[groud]

The previous approach was to make the Godot object initialize the extension object by calling an extension instance constructor. This used to create some issues, with recursive object creation for example (objects constructed into the extension object constructor nowt working).

This solves the issue by reversing the processus, making extension object requesting the corresponding Godot object creation using gdnative_classdb_construct_object, and then registering itself as an instance of this object using gdnative_object_set_instance.

This still need some work, as i am not sure that gdnative_object_set_instance_binding and gdnative_object_get_instance_binding would still be useful.

[akien-mga]

The godot-cpp test build in CI fails, but that's expected, as godotengine/godot-cpp#663 needs to be merged for the test to pass (and this PR here needs to be merged before godotengine/godot-cpp#663 can build...).

So let's merge despite the failed check, this should be fairly exceptional.

[akien-mga]

Thanks!