TF-1900 Add SendQueueIsolateManager to handle sending queue in isolate

(cherry picked from commit 9f65bbec5304d5e50e168706fdde97ec2d16da97)

lib/features/sending_queue/presentation/utils/sending_queue_isolate_manager.dart

@@ -0,0 +1,8 @@
+
+import 'package:tmail_ui_user/features/base/isolate/isolate_manager.dart';
+
+class SendingQueueIsolateManager extends IsolateManager {
+
+  @override
+  String get isolateIdentityName => 'sending_queue_isolate';
+}