Add MuJoCo environments support.

[vwxyzjn]

This PR adds support for MuJoCo environments such as Hopper-v2. Only works for Linux.

poetry install -E mujoco
poetry install -E pybullet
poetry run python cleanrl/ppo_continuous_action.py --gym-id Hopper-v2
poetry run python cleanrl/ppo_continuous_action.py --gym-id HalfCheetahBulletEnv-v0

[dosssman]

Mujoco integration working, tested with ppo_continuous_action.py and sac_continuous_actions.py.

I notice among the file changed that there are quite a few brax related files.
Are those really part and / or relevant to this PR ?

[vwxyzjn]

Yeah the brax stuff comes from another branch... Let me remove them.