TF-1204 Rename Registration to Subscription

fcm/lib/model/firebase_subscription_id.dart

@@ -0,0 +1,13 @@
+
+import 'package:equatable/equatable.dart';
+import 'package:jmap_dart_client/jmap/core/id.dart';
+
+class FirebaseSubscriptionId with EquatableMixin {
+
+  final Id id;
+
+  FirebaseSubscriptionId(this.id);
+
+  @override
+  List<Object?> get props => [id];
+}