Add per-body gravity compensation (buoyancy) passive force.

PiperOrigin-RevId: 485342531
Change-Id: Id9acf63205941ac9ac2f87ec30099bff4ec0f1b7

Author: DeepMind

dm_control/mjcf/schema.xml

@@ -1134,6 +1134,7 @@
             <attribute name="xyaxes" type="array" array_type="float" array_size="6"/>
             <attribute name="zaxis" type="array" array_type="float" array_size="3"/>
             <attribute name="euler" type="array" array_type="float" array_size="3"/>
+            <attribute name="gravcomp" type="float"/>
           </attributes>
           <children>
             <element name="inertial" on_demand="true">