feat(userMgr): add list users endpoint

Add new endpoint `/userMgr/list` to retrieve all users with superuser permission. Includes handler implementation, database query function, and API documentation update.

router.go

@@ -14,6 +14,7 @@ func setupRoutes() {
 
 	http.HandleFunc("/userMgr/create", CreateUserHandler)
 	http.HandleFunc("/userMgr/remove", RemoveUserHandler)
+	http.HandleFunc("/userMgr/list", ListUserHandler)
 
 	http.HandleFunc("/frpcAct/instanceMgr/create", CreateInstanceHandler)
 	http.HandleFunc("/frpcAct/instanceMgr/list", ListInstancesHandler)