refactor(authentication): add dart:async import for future use

- Import dart:async library in request_code_page.dart
- Prepare for potential future implementations that may require Timer or Future functionalities

Author: fulleni

lib/authentication/view/request_code_page.dart

@@ -1,6 +1,8 @@
 //
 // ignore_for_file: lines_longer_than_80_chars
 
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:flutter_news_app_web_dashboard_full_source_code/app/bloc/app_bloc.dart';