Add support for KHR_materials_variants extension

[mdeardorff]

Description

This PR exposes the KHR_materials_variants extension at the Unity level for runtime-switching of textures. Primitives with the extension enabled will automatically add a CesiumMaterialVariants component which can be toggled between variants with SetVariant(int index) or SetVariant(string name).

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

• In Unity, create a CesiumTileset pointed at a tileset with the KHR_materials_variants extension.
• Create a script that does something like:

[SerializeField]
private Cesium3DTileset tileset;

void Update()
{
    if (Keyboard.current.aKey.wasPressedThisFrame)
    {
        if (tileset != null)
        {
            var childVariants = tileset.GetComponentsInChildren<CesiumMaterialVariants>(true);

            foreach (var v in childVariants)
            {

                bool success = v.SetVariant(v.GetCurrentVariantIndex() == 0 ? -1 : 0);
                Debug.Log($"SetVariant result: {success}, now at: {v.GetCurrentVariantName()}");
            }

        }
    }
}

• Expect to see tiles toggle between available variants.