Add control values to keyframes.

- Add basic test for keyframes.
- Add missing documentation for keyframe mocap positions and quaternions.

PiperOrigin-RevId: 459021649
Change-Id: I204cd916d19560fa732bf693b139d98ff415ed85

Author: yuvaltassa

dm_control/mjcf/schema.xml

@@ -2041,6 +2041,9 @@
             <attribute name="qpos" type="array" array_type="float"/>
             <attribute name="qvel" type="array" array_type="float"/>
             <attribute name="act" type="array" array_type="float"/>
+            <attribute name="ctrl" type="array" array_type="float"/>
+            <attribute name="mpos" type="array" array_type="float"/>
+            <attribute name="mquat" type="array" array_type="float"/>
           </attributes>
         </element>
       </children>