fix(core): allow some left menu methods inheritance

part of kestra-io/kestra-ee#4728

Author: brian-mulier-p

ui/src/override/components/useLeftMenu.ts

@@ -259,5 +259,8 @@ export function useLeftMenu() {
         ];
     };
 
-    return {generateMenu};
-}
+    return {
+        routeStartWith,
+        generateMenu
+    };
+}