This adds an option MUJOCO_BUILD_USD_PLUGINS to the top level CMakeLists which allows us to build the usdMjcf file format plugin with the rest of mujoco.
In this first version, we allow building against either standalone USD or against Houdini (DCC) since it has it's own fork of USD. When building against standalone USD we also build the file format tests.
PiperOrigin-RevId: 753969974
Change-Id: If691eecbddf45d18b7c4ff76f7650999d42e99c5
Sam Haves