Create reference implementation for proposed glTF extension EXT_textureInfo_constant_lod

[markschlosseratbentley]

We are currently working on fleshing out support for constant LOD textures in an extension on glTF, found here: CesiumGS/glTF#92.

We should consider adding support for that extension to iTwin.js as it is described at the above link.

[eringram]

Completed: #8882

Gifs/screenshots from PR description:

This is what the effect looks like for this test model:

With a normal map of a bumpy texture: