TF-32 Add domain layer of export attachment for ios platform

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

@@ -0,0 +1,19 @@
+import 'package:core/core.dart';
+
+class ExportAttachmentSuccess extends UIState {
+  final String filePath;
+
+  ExportAttachmentSuccess(this.filePath);
+
+  @override
+  List<Object> get props => [filePath];
+}
+
+class ExportAttachmentFailure extends FeatureFailure {
+  final exception;
+
+  ExportAttachmentFailure(this.exception);
+
+  @override
+  List<Object> get props => [exception];
+}