Updated the interface for all subsystems to apply a consistent pattern across all subsystems. This affects subsystem implementations (such as ARCore and ARKit) but should not affect consumers of those subsystems.
Removed `supported` property on the `XRFaceSubsystem`. Providers (such as ARKit) are expected not to register themselves if they are not supported.
Removed `supported` property on the `XREnvironmentProbeSubsystem`. Providers (such as ARKit) are expected not to register themselves if they are not supported.
