Experimental Godot implementation

Hello! Even though glTFX is not yet finalized, I think it's great, so I wanted to implement it in Godot Engine.

This implementation is not to be considered a source of truth, and is subject to change whenever Khronos changes this repo.

Link: https://github.com/aaronfranke/godot-gltfx-reference-format (this is a plugin that you add to a project, requires Godot Engine 4.4 or later for full features).

Here are some trivial test files generated by my implementation: [gltfx_test_files.zip](https://github.com/user-attachments/files/16365516/gltfx_test_files.zip)

In order to make this, I mostly looked at the old glXF specification, but made the following assumptions:

- Since the format is called "glTF External Reference Format" instead of "glTF Experience Format", I assumed that `"experience": true` would be replaced by `"reference": true`.
- Since the format is now called "glTFX", I assumed that the file extension would be `.gltfx`.
- Since the existing examples do not include `"scene"` and `"scenes"`, I did not do this either. See https://github.com/KhronosGroup/glTF/issues/1542 - multi-scene file formats aren't really a thing, and people have asked for this to be removed from glTF. I don't think glTFX should have this feature if it's being removed from glTF.
- I did not implement the `"scene"` property of asset. See https://github.com/KhronosGroup/glTF/issues/1542 - multi-scene file formats aren't really a thing, and people have asked for this to be removed from glTF. I don't think glTFX should encourage something that people consider a bad idea.

Start by reviewing the experimental implementation linked in the issue and the assumptions listed for the glTFX format, including the .gltfx extension and reference property. The issue does not define a requested change or acceptance criteria, so a contributor would need clarification before work can be considered done.

Written by the indexing model from the issue text.