Initial support for writing UsdPhysics data starting with UsdPhysicsRigidbodyAPI support.

The SDF_FORMAT_ARGS `usdMjcfToggleUsdPhysics` is introduced to toggle writing UsdPhysics or not when converting. Off by default

PiperOrigin-RevId: 746467942
Change-Id: I26cb9a4c4ae2eef96163dab967a74f14940a2a6d

test/experimental/usd/plugins/mjcf/fixture.cc

@@ -32,9 +32,11 @@ namespace mujoco {
 
 using pxr::SdfPath;
 
-pxr::SdfLayerRefPtr LoadLayer(const std::string& xml) {
+pxr::SdfLayerRefPtr LoadLayer(
+    const std::string& xml,
+    const pxr::SdfFileFormat::FileFormatArguments& args) {
   auto layer = pxr::SdfLayer::CreateAnonymous(
-      "test_layer", pxr::SdfFileFormat::FindByExtension("xml"));
+      "test_layer", pxr::SdfFileFormat::FindByExtension("xml"), args);
   layer->ImportFromString(xml);
   EXPECT_THAT(layer, testing::NotNull());
   return layer;