Allow changing marker/keypoint site render radius in mujoco_viz()

[charles-zhng]

Currently, the radius for the spheres for marker and keypoints sites are set to 0.005m. For smaller models like the mouse model, this is too large. So add an optional argument to the create_site functions and in mujoco_viz() so that a user defined radius value can be passed.

https://github.com/talmolab/stac-mjx/blob/main/stac_mjx/controller.py#L67
https://github.com/talmolab/stac-mjx/blob/main/stac_mjx/controller.py#L113
https://github.com/talmolab/stac-mjx/blob/main/stac_mjx/viz.py#L312