Add mjs_attachFrameToSite.

This is required in preparation of changing the spec.attach behavior in the Python bindings.

PiperOrigin-RevId: 730810198
Change-Id: I095808f0afe62b5bbd27201ed5c75804e0e9c4bf

include/mujoco/mujoco.h

@@ -1426,6 +1426,10 @@ MJAPI mjsFrame* mjs_attachFrame(mjsBody* parent, const mjsFrame* child,
 MJAPI mjsBody* mjs_attachToSite(mjsSite* parent, const mjsBody* child,
                                 const char* prefix, const char* suffix);
 
+// Attach child frame to a parent site, return the attached frame if success or NULL otherwise.
+MJAPI mjsFrame* mjs_attachFrameToSite(mjsSite* parent, const mjsFrame* child,
+                                      const char* prefix, const char* suffix);
+
 // Detach body from mjSpec, remove all references and delete the body, return 0 on success.
 MJAPI int mjs_detachBody(mjSpec* s, mjsBody* b);