Extension support is incomplete

I might be holding this wrong, but it looks like support for extensions is incomplete and documentation around extensions is lacking.

There is an ExtensionFactory and IExtension interface exposed, and users can register their own callbacks for extensions; however, these seem to have only exposed a Serialize() and Deserialize() method to read/write the extension data - it seems there's no callback to associate the data with a GameObject (i.e. to do anything _useful_ with the extension data.)

The GLTFSceneImporter class has hardcoded a specially-privileged "MSFT_LOD" extension - this is an indication that the extension support is not good enough. There should be sufficient callbacks in the importer to do this without having to modify the importer.

At the very least, there should be a sample which shows how users can register and use custom extensions to add their own MonoBehaviour to GameObjects.