Use gltf extension in Godot for Actors

[CarlCochet]

This PR is a first attempt at structuring the project to use the various custom glTF extensions used in the lib.

As a PoC, it aims at implementing some Actors functionalities to be able to simulate dynamic scenes within the Godot engine through the simulation stepping process.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.