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
PiperOrigin-RevId: 789385654 Change-Id: Ib15450e81258e3d202d8123c0c43f8421ebefed6
PiperOrigin-RevId: 789365870 Change-Id: I19cce03ec2ffc28cdf4252bfd02c8b5163bb5ba2