Add support for KHR_XMP extension

[pjcozzi]

This extension is still in draft and under active development. It will be used to store credits/copyright/etc for models, including parts of models such as individual meshes and textures. Once this extension is finished, gltf-pipeline should aim to preserve this content when an input glTF contains it. This will involve following some TBA best practices for when meshes and textures are merged, removed, etc.

[pjcozzi]

See https://github.com/KhronosGroup/glTF/pull/1553