Add ability to specify a material assignment to a mesh asset.

This extends both the spec and the XML definition to allow specification of a material name on a mesh. This material acts as a fallback in the case that the referencing geom element does not specify a material of it's own.

PiperOrigin-RevId: 798232378
Change-Id: Ibac3264b79b9f2192910853ca8caf3ce49fb738e

Author: DeepMind

dm_control/mjcf/schema.xml

@@ -960,6 +960,7 @@
             <attribute name="inertia" type="keyword" valid_values="legacy convex exact shell"/>
             <attribute name="builtin" type="keyword" valid_values="wedge rect"/>
             <attribute name="parameters" type="array" array_type="float"/>
+            <attribute name="material" type="reference"/>
           </attributes>
           <children>
             <element name="plugin" repeated="true" namespace="mesh">