refactored alarm models

lib/model/callback_models/create_relative_time_alarm_request.dart

@@ -0,0 +1,9 @@
+import '../alarm.dart' show AlarmType;
+import 'create_alarm_request.dart';
+
+class CreateRelativeTimeAlarmRequest extends CreateAlarmRequest {
+  final DateTime? triggerAt;
+
+  CreateRelativeTimeAlarmRequest({this.triggerAt})
+    : super(alarmType: AlarmType.timeRelative);
+}