Planes platform support

Understand the AR Foundation capabilities implemented by the OpenXR Spatial Entities plane provider.

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

Feature	Descriptor Property	Supported
Horizontal plane detection	supportsHorizontalPlaneDetection	Yes
Vertical plane detection	supportsVerticalPlaneDetection	Yes
Arbitrary plane detection	supportsArbitraryPlaneDetection	Yes
Boundary vertices	supportsBoundaryVertices	Determined at runtime
Classification	supportsClassification	Determined at runtime