Add mjs_attachFrame to user_api.

PiperOrigin-RevId: 625648309
Change-Id: I4d6dfc4d38ad6ce688df83eb7cc2c34676076193

src/user/user_objects.h

@@ -275,6 +275,7 @@ class mjCBody : public mjCBody_, private mjsBody {
 
   // API for adding existing objects to body
   mjCBody& operator+=(const mjCBody& other);
+  mjCBody& operator+=(const mjCFrame& other);
 
   // API for accessing objects
   int NumObjects(mjtObj type);
@@ -360,7 +361,6 @@ class mjCFrame : public mjCFrame_, private mjsFrame {
   void SetParent(mjCBody* _body);
 
   mjCFrame& operator+=(const mjCBody& other);
-  mjCFrame& operator+=(const mjCFrame& other);
 
  private:
   mjCFrame(mjCModel* = 0, mjCFrame* = 0);      // constructor