TF-32 Add domain layer of download attachment for android platform

lib/features/email/domain/state/download_attachments_state.dart

@@ -0,0 +1,20 @@
+import 'package:core/core.dart';
+import 'package:model/model.dart';
+
+class DownloadAttachmentsSuccess extends UIState {
+  final List<DownloadTaskId> taskIds;
+
+  DownloadAttachmentsSuccess(this.taskIds);
+
+  @override
+  List<Object> get props => [taskIds];
+}
+
+class DownloadAttachmentsFailure extends FeatureFailure {
+  final exception;
+
+  DownloadAttachmentsFailure(this.exception);
+
+  @override
+  List<Object> get props => [exception];
+}