Meta Quest support

[nlsdvl]

As the Meta Quest is technically an Android device, features available in the project require testing on that platform:

• AR anchors, including anchoring to controllers and hands
• interactivity reacting to XR controllers triggers
• media pipelines

The Meta Quest platform provides tracking features needed to implement Avatar features.

[nlsdvl]

Meta Quest support is demonstrated in the feature/meta-quest with the current limitation that its not possible to switch between demos (only a single glTF document).

What's done:

• Upgrade the project and dependencies to Unity 6
• Implement interactivity triggers for the XR Quest (MPEG_scene_interactivity)
• Passthrough mode is working

What's missing:

• Documentation
• Create build profiles for both OpenXR and ARCore
• Add a menu to switch between scenes
• Implement XR controller anchoring (MPEG_anchor)
• Configure whether the HMD works in passthrough mode

cc @haudiobe @jordijoangimenez