Support glTF extension: KHR_materials_variants

[mvaligursky]

https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_variants/README.md

This extension allows for a compact glTF representation of multiple material variants of an asset, structured to allow low-latency switching at runtime.

A typical use case is digital commerce, where a user might be presented with e.g. a pair of sneakers and the ability to switch between different colours.