Add two new attributes to weld constraints:
Add two new attributes to weld constraints:

- `anchor` determines the point of wrench application, in the frame of body2.
- `tfratio` scales applied torques relative to applied forces.
- Add visualisation of both anchor points to both weld and connect constraints.
- Add a test model showing how the new weld parameters behave.

PiperOrigin-RevId: 469190483
Change-Id: Idf5928fbf278fb10357a77d2de2455d4a525d95a

deepmind/dm_control