Support adding basic primatives

[tancik]

We should make it easy to add basic primitives like cubes and spheres.

[brentyi]

It's pretty quick to generate these with trimesh and then send them over, like here:
https://github.com/nerfstudio-project/viser/blob/5bd1d2e7a60c64011dd51a8db8d240d52848b5d7/examples/12_click_meshes.py#L45C1-L64

Do you think we should write our own wrappers? Maybe just highlighting this in the documentation would be enough.

[tancik]

I think wrappers could be nice. These are very common actions, requiring people to use trimesh requires knowing another library. Additionally, tab completing viser_server.add_ to see all the things you can add will probably be a pretty common user flow, having add_cube show up right there would be handy.