USD (and glTF) physics rigid bodies export/import support

USD supports rigid bodies and works on deformable bodies. GLTF with upcoming extensions adds support too and there is experimental external Blender importer/exporter github.com/eoineoineoin/glTF_Physics_Blender_Exporter that shows how this can work in Blender.

Adding this support would make Blender a little more popular choice especially in game dev.

Mentioned GLTF exporter adds some additional UI options that Blender doesn't have by default to properly cover support. USD/GLTF physics rigid bodies should be compatible so if we add support for 1 file format it will be easier to add another.