Add support for anisotropy in RealityKit converter

[warrenm]

Thanks to the KHR_materials_anisotropy extension and cgltf v1.14, we have first-class support for anisotropy in the GLTFAsset API, but this isn't yet wired up in RealityKit material import.

The task is to figure out how to map the extension's anisotropy_strength and anisotropy_rotation properties (and texture) onto RealityKit's PhysicallyBasedMaterial.AnisotropyLevel and PhysicallyBasedMaterial.AnisotropyAngle and implement the feature in the RealityKit converter.