Add support for parsing PhysX spatial tendons from USD for integration testing

[nv-rgresia]

In order to test spatial tendons with Isaac Lab via Selection API, the tendons have to be defined within an USD scene so they can be included in an environment configuration. This can be achieved by extending the USD importer to parse PhysX spatial tendons and converting them into their Mujoco equivalent.

[preist-nvidia]

We will not continue down the PhysX schema compatible route but instead work on mjcPhysics/MJCF defined spatial tendons.