Add usd_decoder to mujoco/plugin.
- Previously was src/experimental/usd/usd_to_mjspec.cc, now that same spec is returned by the plugin.
PiperOrigin-RevId: 854350962
Change-Id: Ia980190a557c50ce8197980922a2594b613859b4
Still missing compiler settings but will follow up in a future CL. Tried to add full coverage for testing setting of attributes to non-default values.
PiperOrigin-RevId: 761636862
Change-Id: Id4b9486645b2600931556cdcb8dc71f28ec309cd