Support registering additional GLTF extensions

[merlosy]

Description

I am currently trying to build a small project to try to extend the model-viewer to support more gltf extensions on load (MSFT_audio_emitter).
I had some challenges on several aspects:

• Doing some clean up while unregistering a GLTF extension callback. It involves keeping a reference to the extension instance to be able to call some "dispose/destroy" method (in my case to call URL.revokeObjectURL). Maybe there is a hook/event that can be used to do it more cleanly?
• Make the ModelScene extendable to add extra methods and/or properties

The plan is to be fully backward compatible. Is it something that you are open to?

The other option would be to open a PR to integrate it natively (like in Babylonjs) but I think it would miss an opportunity for people to build on top this great project.

Live Demo

Nothing working at this stage

Version

• model-viewer: v1.10.1

Browser Affected

• Chrome, version: xx.x.xxxx.xx
• Edge
• Firefox
• IE
• Safari

OS

• Android
• iOS
• Linux
• MacOS
• Windows

AR

• WebXR
• SceneViewer
• QuickLook

[elalish]

Thank you, PRs certainly welcome! ModelScene is not part of our public API, so that might not be the best place to add things (though I may have misunderstood your intention). You may want to follow our other loader APIs like dracoDecoderLocation; note they are static, since the loader is shared among MV elements, and their style allows them to be set before the element starts pulling down the src, which is often pretty immediate.

[merlosy]

I'd like to start working on this if no one does in the meantime :)
The only constraint is that I need to find some time.