feat: use batch download api for create multitasks (#978)

ui/flutter/lib/api/model/create_task_batch.dart

@@ -0,0 +1,39 @@
+import 'package:json_annotation/json_annotation.dart';
+
+import 'options.dart';
+import 'request.dart';
+
+part 'create_task_batch.g.dart';
+
+@JsonSerializable(explicitToJson: true)
+class CreateTaskBatch {
+  List<CreateTaskBatchItem>? reqs;
+  Options? opt;
+
+  CreateTaskBatch({
+    this.reqs,
+    this.opt,
+  });
+
+  factory CreateTaskBatch.fromJson(
+    Map<String, dynamic> json,
+  ) =>
+      _$CreateTaskBatchFromJson(json);
+
+  Map<String, dynamic> toJson() => _$CreateTaskBatchToJson(this);
+}
+
+@JsonSerializable()
+class CreateTaskBatchItem {
+  Request? req;
+  Options? opts;
+
+  CreateTaskBatchItem({
+    this.req,
+    this.opts,
+  });
+
+  factory CreateTaskBatchItem.fromJson(Map<String, dynamic> json) =>
+      _$CreateTaskBatchItemFromJson(json);
+  Map<String, dynamic> toJson() => _$CreateTaskBatchItemToJson(this);
+}