Add vertex equality option.

PiperOrigin-RevId: 860299454
Change-Id: Iee436fbe4defd10750db1a63d2f36e7f57f33884

Author: quagla

dm_control/mjcf/schema.xml

@@ -1895,6 +1895,16 @@
             <attribute name="flex" type="identifier"/>
           </attributes>
         </element>
+        <element name="flexvert" repeated="true" namespace="equality">
+          <attributes>
+            <attribute name="name" type="identifier"/>
+            <attribute name="class" type="reference" reference_namespace="default"/>
+            <attribute name="active" type="keyword" valid_values="false true"/>
+            <attribute name="solref" type="array" array_type="float" array_size="2"/>
+            <attribute name="solimp" type="array" array_type="float" array_size="5"/>
+            <attribute name="flex" type="identifier"/>
+          </attributes>
+        </element>
       </children>
     </element>
     <element name="tendon">