924ee3070a
For the MJCF -> USD plugin, and I suspect other usecases for user values, it's necessary to give ownership of the object to Mujoco. However since they get type erased, we can't clean up the data automatically for the user. Instead this allows c++ clients to provide a cleanup function with their data. PiperOrigin-RevId: 756832010 Change-Id: I80b8e7822e1a0e399a0d19dcaa57ee69b3ccc16a