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

model/lib/download/download_task_id.dart

@@ -0,0 +1,10 @@
+import 'package:equatable/equatable.dart';
+
+class DownloadTaskId with EquatableMixin {
+  final String taskId;
+
+  DownloadTaskId(this.taskId);
+
+  @override
+  List<Object> get props => [taskId];
+}