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: Iaf300c727732744ea1b613e64334ee505ac6e209

python/mujoco/introspect/structs.py

@@ -9353,6 +9353,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjsPlugin'),
                  doc='sdf plugin',
              ),
+             StructFieldDecl(
+                 name='material',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjString'),
+                 ),
+                 doc='name of material',
+             ),
              StructFieldDecl(
                  name='info',
                  type=PointerType(