Parse closed kinematic chains from SDF (and perhaps also URDF)

[RussTedrake]

SAP now supports constraints, but currently users can only define those constraints programmatically.

Closed kinematic chains are a common feature in robots (e.g. Cassie, or in many underactuated hands). We should support parsing a robot with a closed kinematic chain directly from SDF and/or URDF. The specification in URDF is such that one cannot define a closed chain without additional tags, but in SDF it is possible to define a closed chain just by adding the already defined links and joints. Presumably we throw an error in this case, but I have not verified that.

We used to have a Drake-specific "loop_joint" tag. You can still find vestiges of it in our codebase; the URDF parser even reports Error(*node, "loop joints are not supported in MultibodyPlant"); if it encounters one. I believe @sherm1 feels that it's better for the user to be able to decide where the mechanism should be split and closed with a constraint (vs the true joints), and that it's better to split in the middle of a link and then have a constraint which welds the link back together (vs a lower-DOF constraint implemented at a joint). I am ok with requiring a custom tag.

A victory condition here would be to have the four_bar.sdf and even strandebeest use constraints without having to add them programmatically. They should be specified in the robot description file.