Use a sample MJCF asset featuring spatial tendons and add support for MJCF import of spatial tendon sites and definitions, and then confirm that Newton API control position targets can be set on spatial tendon actuator definitions to actuate the sample.

For all parsing subtasks, please add corresponding UTs to https://github.com/newton-physics/newton/blob/main/newton/tests/test_import_mjcf.py

For the control subtask, please add a tendon UT file.