Anchors platform support

This package implements the following optional features of AR Foundation's XRAnchorSubsystem:

Feature	Descriptor Property	Supported
Trackable Attachment	supportsTrackableAttachments
Synchronous Add Anchor	supportsSynchronousAdd
Save Anchor	supportsSaveAnchor	Yes
Load Anchor	supportsLoadAnchor	Yes
Erase Anchor	supportsEraseAnchor	Yes
Get Saved Anchor Ids	supportsGetSavedAnchorIds
Async Cancellation	supportsAsyncCancellation

Shared anchors

This package also provides functionality to share anchors between devices. The implementation of shared anchors is specific to this platform. To learn more about this feature, refer to Shared anchors.