Add Contextual Heinrich Env

[TheEimer]

The Go2 is in the Mujoco Menagerie, so this would be interesting since we wanted Mujoco anyway/need to see how that relates to brax.

Useful links:

• https://github.com/google-deepmind/mujoco_menagerie/tree/main/unitree_go2
• https://github.com/google-deepmind/mujoco_playground/blob/main/mujoco_playground/experimental/learning/handstand.ipynb