refactored alarm models

lib/model/relative_time_alarm.dart

@@ -0,0 +1,38 @@
+import 'alarm.dart';
+
+class RelativeTimeAlarm implements Alarm {
+  @override
+  final String id;
+
+  @override
+  final String taskId;
+
+  final DateTime triggerAt;
+
+  @override
+  final AlarmType alarmType;
+
+  const RelativeTimeAlarm({
+    required this.id,
+    required this.taskId,
+    required this.triggerAt,
+  }) : alarmType = AlarmType.timeRelative;
+
+  factory RelativeTimeAlarm.fromJson(Map<String, dynamic> json) {
+    return RelativeTimeAlarm(
+      id: json['id'] as String,
+      taskId: json['taskId'] as String,
+      triggerAt: DateTime.parse(json['triggerAt'] as String),
+    );
+  }
+
+  @override
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'taskId': taskId,
+      'triggerAt': triggerAt.toIso8601String(),
+      'alarmType': alarmType,
+    };
+  }
+}