Add "cameraid" attribute to visual/global specifying the default camera to use at model load.

PiperOrigin-RevId: 773783069
Change-Id: I3d6db983273994b7649204bb06f977b2feab56ee

Author: yuvaltassa

dm_control/mjcf/schema.xml

@@ -123,6 +123,7 @@
       <children>
         <element name="global">
           <attributes>
+            <attribute name="cameraid" type="int"/>
             <attribute name="fovy" type="float"/>
             <attribute name="ipd" type="float"/>
             <attribute name="azimuth" type="float"/>