Add attach tag to MJCF.

PiperOrigin-RevId: 651117953
Change-Id: I57b0e53ed44607d479b3785d908e0837c0804cf3

src/user/user_api.h

@@ -60,6 +60,9 @@ MJAPI void mj_copyBack(mjSpec* s, const mjModel* m);
 // Delete spec.
 MJAPI void mj_deleteSpec(mjSpec* s);
 
+// Add spec (model asset) to spec.
+MJAPI void mjs_addSpec(mjSpec* s, mjSpec* child);
+
 
 //---------------------------------- Attachment ----------------------------------------------------
 
@@ -182,6 +185,9 @@ MJAPI mjsMaterial* mjs_addMaterial(mjSpec* s, mjsDefault* def);
 // Get spec from body.
 MJAPI mjSpec* mjs_getSpec(mjsBody* body);
 
+// Find spec (model asset) by name.
+MJAPI mjSpec* mjs_findSpec(mjSpec* spec, const char* name);
+
 // Find body in model by name.
 MJAPI mjsBody* mjs_findBody(mjSpec* s, const char* name);