Better feature handling

[RaananW]

A feature that is a native WebXR feature will not attach if the session was not enabled with it, or that the session doesn't support it.

In addition, a warning will be shown if the feature was initialized after the session has started.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/14732/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14732/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14732/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/14732/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/14732/merge
https://gui.babylonjs.com/?snapshot=refs/pull/14732/merge
https://nme.babylonjs.com/?snapshot=refs/pull/14732/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/14732/merge#BCU1XR#0