FBXDocument register/unregister document extensions are non functional

### Tested versions

- Reproducable in https://github.com/godotengine/godot/commit/f2045ba822bff7d34964901393581a3117c394a9
- not Reproducable in dev3

The FBXDocument is a new feature

### System information

Windows 11, Nvidia 3090, Vulkan forward+, Vulkan mobile

### Issue description

During the merge of FBXDocument, the GLTFExtension system was not implemented, commented stubs were left in c++ where the extensions should go.

### Steps to reproduce

I have not executed this sample code code.

```gdscript
var fbx = FBXDocument.new()
fbx.register_gltf_document_extension # ...
fbx.append_from_file # ...
fbx.generate_scene # ...
```

### Minimal reproduction project (MRP)

The bug is visible in a brand new project.

Start by inspecting the FBXDocument implementation and the commented C++ extension stubs described in the issue. Reproduce the failure with the shown GDScript flow using register_gltf_document_extension, append_from_file, and generate_scene; done means document extension registration and unregistration function through that flow.

Written by the indexing model from the issue text.