Static Magnet for Connectors

First of all, thank you very much for all contributors to this MuJoCo project. This is an awesome repository.

**Is your feature request related to a problem? Please describe.**

I want to make a magnet to implement something like [MagSafe 3](https://www.apple.com/shop/product/MPL23AM/A/usb-c-to-magsafe-3-cable-2-m-space-gray) of Mac.

Implementing magnetic forces might be useful for other functions too. For example, adhesion currently attracts all objects within margin - gap with the same force strength. However, if one needs a realistic suction gripper, its force strength needs to depend on the distance between the gripper and an object, in a way similar to magnetic forces. (Closer, stronger.)

**Describe the solution you'd like**

It would be nice if we can add to an object a sclar attribute (like "magnetism"), which determines the magnitude of the force, and another attribute that determines the orientation of the magnetism (S and N). It would be also nice if setting the orientation to some special value allows to instantiate something like metal-like objects (no repulsion, no attraction of other objects, only attraction to magnets) and gravitation (no repulsion, only attraction). That said, for the purpose I descibed above (i.e., MagSafe 3), ferromagnetism and magnetic forces caused by electric currents are unnesesary.

**Describe alternatives you've considered**

Currently, I have no other idea on how to implement magnets. Setting contype and conaffinity with adhesion actuators allows an ad-hoc method to implement something like magnets. However, it sometimes leads to no contacts between some objects.

**Additional context**

I opened an issue about this topic, and Yuval asked me to open a feature request. See this issue for the additional context: https://github.com/deepmind/mujoco/issues/615#issuecomment-1332673517