Implement WebXR's secondary views feature

[RaananW]

See - https://www.w3.org/TR/webxr/#secondary-view

This should technically replace spectator mode (or be used under the hood)

[mr-sudo]

Here's a link to the related BabylonJS forum bug report for recording AR experiences on Hololens 2
https://forum.babylonjs.com/t/recording-babylonjs-scene-on-hololens-2/34325

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