Add support for TRACKABLE_ANCHOR_3D

[nlsdvl]

Currently TRACKABLE_ANCHOR_3D is parsed by not instantiated by the XR Player.

The Object tracking Unity feature seems to be matching the expected functionnality, but it relies on objects being in plateform / provider specific formats, where TRACKABLE_ANCHOR_3D references a 3D node in a glTF document with MPEG_anchor extension.

The provider specific format might not be serializable to a standard glTF node, for instance if it includes descriptors that are not standard geometry.

• XRReferenceObjectEntry

A reference object represents a 3D scan of a real object that can be recognized in the environment. Each XRReferenceObject contains a list of provider-specific XRReferenceObjectEntry. Each provider (implementation of XRObjectTrackingSubsystem) should derive a new type from this type.

Each XRReferenceObjectEntry is generally an asset on disk in a format specific to that provider.

Currently this feature is not supported on Android : https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@6.2/manual/features/object-tracking.html?q=XRReferenceObject