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

include/mujoco/mjspec.h

@@ -480,6 +480,7 @@ typedef struct mjsMesh_ {          // mesh specification
   mjIntVec* userface;              // user vertex indices
   mjIntVec* userfacetexcoord;      // user texcoord indices
   mjsPlugin plugin;                // sdf plugin
+  mjString* material;              // name of material
   mjString* info;                  // message appended to compiler errors
 } mjsMesh;