Add mjContact.solreffriction, mjModel.pair_solreffriction and related implementation, allowing contact reference acceleration to be different for normal and frictional contact directions. This is required for e.g. elastic frictional collisions.

PiperOrigin-RevId: 538513357
Change-Id: Ic91f34cbc143b7edd33e4b68d29e471f2db9e912

Author: yuvaltassa

dm_control/mjcf/schema.xml

@@ -351,6 +351,7 @@
             <attribute name="condim" type="int"/>
             <attribute name="friction" type="array" array_type="float" array_size="5"/>
             <attribute name="solref" type="array" array_type="float" array_size="2"/>
+            <attribute name="solreffriction" type="array" array_type="float" array_size="2"/>
             <attribute name="solimp" type="array" array_type="float" array_size="5"/>
             <attribute name="margin" type="float"/>
             <attribute name="gap" type="float"/>
@@ -1422,6 +1423,7 @@
             <attribute name="condim" type="int"/>
             <attribute name="friction" type="array" array_type="float" array_size="5"/>
             <attribute name="solref" type="array" array_type="float" array_size="2"/>
+            <attribute name="solreffriction" type="array" array_type="float" array_size="2"/>
             <attribute name="solimp" type="array" array_type="float" array_size="5"/>
             <attribute name="margin" type="float"/>
             <attribute name="gap" type="float"/>