TF-1289: [Domain] add storeSubscription in fcm repository

lib/features/push_notification/domain/model/fcm_subscription.dart

@@ -0,0 +1,16 @@
+
+import 'package:equatable/equatable.dart';
+
+class FCMSubscription with EquatableMixin {
+
+  final String deviceId;
+  final String subscriptionId;
+
+  FCMSubscription(this.deviceId, this.subscriptionId);
+
+  @override
+  List<Object?> get props => [
+    deviceId,
+    subscriptionId
+  ];
+}