Support pure force elements?

This came up in discussion on an internal Slack discussion.

We have a dishwasher model that has a linear spring between the door and the body.
We could add another joint, but that's a "junk" DOF, and Drake does not yet support closed-loop topologies for DOFs.

However, we do support force elements in the Drake API.
Would be nice to have a "force element" class of objects that could be added to the spec, e.g. like Drake's LinearSpringDamper.