This is useful for custom tagging of objects with properties (for example in Blender) and having this available in the editor for scripting.
- Adds import logic to propagate the parsed GLTF extras all the way to the resulting Node->meta
- Adds export logic to save Godot Object meta into GLTF extras
- Supports `nodes`, `meshes` and `materials` (in GLTF sense of the words)
