[XR] Introduce support for WebXR Layer API

[RaananW]

Following the specs - https://www.w3.org/TR/webxrlayers-1/
Explainer - https://github.com/immersive-web/layers/blob/main/explainer.md

WebXR layers should be introduced as an XR feature.

Still TBD:

• What would be the fallback to systems not supporting this? A polyfill but that depends on the dev integrating it.
• Should we introduce a new WebXRVideoTexture (to support video layers) or can we adjust the video texture accordingly?
• Will (or should) it be a single feature or a few features using the same underlying API?
• Are there any changes required to the rendering pipeline? Any changes required to the engine itself?

[gabrieljbaker]

Great to see an issue for this @RaananW !

[RaananW]

The work is currently in progress, hope to have a testable version soon

[thomlucc]

Thanks @rgerd !