Cube unscramble task

[thowell]

Task to unscramble Rubik's Cube (Kevin Zakka model).

Notes:

• the damping is decreased a bit to make the task easier

• there is an additional "transition_model" that is used by Task::TransitionLocked to simulate a cube in order to generate a random scramble

• [x] Fix initial scene view @yuvaltassa

• [x] Share shadow hand model and assets

https://github.com/google-deepmind/mujoco_mpc/assets/12521315/25ff4d5a-5b51-46f0-b34c-ec8d699a0a45

[kevinzakka]

Amazing!