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.

[j9liu]

Thank you so much for the contribution @mdeardorff! We'll take a look at this when we get the chance 🙏

[mdeardorff]

Previously the material variants would just use the default Cesium tileset material. I've made it so it will now use the opaqueMaterial passed into the tileset instead (or the default from UnityPrepareRendererResources if not present).

Also, I'm curious if anyone has any ideas on whether the variants should be exposed at the tileset level instead of just the primitive. It's not very ergonomic right now to iterate through all primitives and do primitive.SetVariant(0) as opposed to tileset.SetVariants(0). Though my understanding of the extension is that controlling all of them at once is technically incorrect/undefined.