Occlusion platform support

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

Note

The Meta Quest Occlusion feature is currently in pre-release and does not yet support environment depth image capabilities.

Feature	Descriptor Property	Supported
Environment Depth Confidence Image	environmentDepthConfidenceImageSupported
Environment Depth Image	environmentDepthImageSupported
Environment Depth Temporal Smoothing	environmentDepthTemporalSmoothingSupported
Human Segmentation Depth Image	humanSegmentationDepthImageSupported
Human Segmentation Stencil Image	humanSegmentationStencilImageSupported