MJX: add differentiable distance calculation

### The feature, motivation and pitch

I am working on a problem where I need to differentiably calculate the distance between two convex objects, including when they are _not_ colliding. For many of the collision algorithms, the distance is hard-set to 1 when there is no contact, breaking the gradient chain, and the function `mj_geomDistance` does not appear to exist in MJX (unless I am missing something).

Either porting `mj_geomDistance`, or setting a flag that lets the collision algorithm leave the positive distance in place rather than hard-setting it to 1, would be appreciated.

### Alternatives

Currently, since one of the objects is a sphere, I manually edit https://github.com/google-deepmind/mujoco/blob/82e92cbcaae55b381a34de58be84b5a3e8c18093/mjx/mujoco/mjx/_src/collision_convex.py#L217 to `dist = d - sphere.size[0]`.

I have no idea how this might break the simulation (other collision algorithms currently set distance to 1 whenever there is no contact). It has not caused me trouble so far.

Note I also need the "contact" normal, but the frame is set appropriately even when there is positive signed distance.

### Additional context

_No response_