TF-1204 Rename Registration to Subscription

fcm/lib/model/firebase_capability.dart

@@ -0,0 +1,8 @@
+
+import 'package:jmap_dart_client/jmap/core/capability/capability_identifier.dart';
+
+class FirebaseCapability extends CapabilityIdentifier {
+  static final identifier = FirebaseCapability(Uri.parse('com:linagora:params:jmap:firebase:push'));
+
+  FirebaseCapability(super.value);
+}