Support user-defined cameras in mujoco.loaders

[flferretti]

This pull request will add support for user-defined cameras when using a mujoco.loaders instance. In particular, the user is required to pass a dictionary or a list of dictionaries containing 5 attributes, e.g.:

camera = dict(
    name="cartpole_camera",
    mode="fixed",
    pos="3.954 3.533 2.343",
    xyaxes="-0.594 0.804 -0.000 -0.163 -0.120 0.979",
    fovy="60",
)

The camera can be then used inside the visualizers passing the argument camera_name.

recorder.record_frame(camera_name="cartpole_camera")

---

📚 Documentation preview 📚: https://jaxsim--133.org.readthedocs.build//133/

[diegoferigo]

The solution unpacking a dict is quite fragile, but for the moment it works fine.

[traversaro]

fyi @CarlottaSartore @giotherobot @siddharthdeore that could be interested in this, xref: robotology/icub-models#155 .