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.h

@@ -21,6 +21,7 @@
 #include "test/fixture.h"
 #include <pxr/usd/sdf/assetPath.h>
 #include <pxr/usd/sdf/declareHandles.h>
+#include <pxr/usd/sdf/fileFormat.h>
 #include <pxr/usd/sdf/path.h>
 #include <pxr/usd/usd/common.h>
 #include <pxr/usd/usd/modelAPI.h>
@@ -34,6 +35,16 @@
     EXPECT_TRUE((stage)->GetPrimAtPath(SdfPath(path)).IsA<type>()); \
   }
 
+#define EXPECT_PRIM_API_APPLIED(stage, path, api)                     \
+  {                                                                   \
+    EXPECT_TRUE((stage)->GetPrimAtPath(SdfPath(path)).HasAPI<api>()); \
+  }
+
+#define EXPECT_PRIM_API_NOT_APPLIED(stage, path, api)                  \
+  {                                                                    \
+    EXPECT_FALSE((stage)->GetPrimAtPath(SdfPath(path)).HasAPI<api>()); \
+  }
+
 #define EXPECT_PRIM_KIND(stage, path, kind)                          \
   {                                                                  \
     pxr::TfToken prim_kind;                                          \
@@ -51,7 +62,9 @@
   }
 namespace mujoco {
 
-pxr::SdfLayerRefPtr LoadLayer(const std::string& xml);
+pxr::SdfLayerRefPtr LoadLayer(
+    const std::string& xml,
+    const pxr::SdfFileFormat::FileFormatArguments& args = {});
 
 template <typename T>
 void ExpectAttributeEqual(pxr::UsdStageRefPtr stage, const char* path,