add pin login and add pin code

lib/constants.dart

@@ -83,6 +83,7 @@ const privilege_receiving = "rc";
 const privilege_pickup = "pku";
 const privilege_collect = "col";
 const privilege_report = "rpt";
+const privilege_pin ="pin";
 
 // Pickup types
 const shipment_local_pickup = "Local pickup";