TF-1068 add abstraction for login username

lib/features/login/data/datasource/login_username_datasource.dart

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+import 'package:tmail_ui_user/features/login/domain/model/recent_login_username.dart';
+
+abstract class LoginUsernameDataSource {
+  Future<void> saveLoginUsername(RecentLoginUsername username);
+
+  Future<List<RecentLoginUsername>> getAllRecentLoginUsernamesLatest({int? limit, String? pattern});
+}

lib/features/login/domain/repository/login_username_repository.dart

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+import 'package:tmail_ui_user/features/login/domain/model/recent_login_username.dart';
+
+abstract class LoginUsernameRepository {
+  Future<void> saveLoginUsername(RecentLoginUsername recentLoginUsername);
+
+  Future<List<RecentLoginUsername>> getAllRecentLoginUsernameLatest({int? limit, String? pattern});
+}