SDF <-> USD: Convert <collision><surface><friction> to USD Material and viceversa

[ahcorde]

By able to convert <collision><surface><friction>

<collision name='back_collision'>
   <pose relative_to="base_link"> -0.364 0 0.373 0 0 0 </pose>
   <geometry>
     <box>
       <size>0.048 0.705 0.186</size>
       </box>
   </geometry>
   <surface>
     <friction>
       <ode>
         <mu>0.6</mu>
         <mu2>0.6</mu2>
       </ode>
     </friction>
   </surface>
 </collision>

To USD Materials

       rel material:binding:physics = </World/PhysicsMaterial> (
           bindMaterialAs = "weakerThanDescendants"
       )


    def Material "PhysicsMaterial" (
       prepend apiSchemas = ["PhysicsMaterialAPI", "PhysxMaterialAPI"]
   )
   {
       float physics:density = 1
       float physics:dynamicFriction = 0.6
       float physics:restitution = 0.4
       float physics:staticFriction = 0.8
       uniform token physxMaterial:frictionCombineMode = "min"
       uniform token physxMaterial:restitutionCombineMode = "min"
   }

[ahcorde]

PR gazebosim/sdformat#975