Add MjcPhysicsSceneAPI support.

Still missing compiler settings but will follow up in a future CL. Tried to add full coverage for testing setting of attributes to non-default values.

PiperOrigin-RevId: 761636862
Change-Id: Id4b9486645b2600931556cdcb8dc71f28ec309cd

test/experimental/usd/test_utils.cc

@@ -44,9 +44,9 @@ pxr::SdfLayerRefPtr LoadLayer(
 
 template <>
 void ExpectAttributeEqual<pxr::SdfAssetPath>(pxr::UsdStageRefPtr stage,
-                                             const char* path,
+                                             pxr::SdfPath path,
                                              const pxr::SdfAssetPath& value) {
-  auto attr = stage->GetAttributeAtPath(pxr::SdfPath(path));
+  auto attr = stage->GetAttributeAtPath(path);
   EXPECT_TRUE(attr.IsValid());
   pxr::SdfAssetPath attr_value;
   attr.Get(&attr_value);