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

Author: DeepMind

dm_control/mjcf/schema.xml

@@ -1398,6 +1398,8 @@
             <attribute name="body1" type="reference" required="true" reference_namespace="body"/>
             <attribute name="body2" type="reference" reference_namespace="body"/>
             <attribute name="relpose" type="array" array_type="float" array_size="7"/>
+            <attribute name="anchor" type="array" array_type="float" array_size="3"/>
+            <attribute name="tfratio" type="float"/>
           </attributes>
         </element>
         <element name="joint" repeated="true" namespace="equality">