TF-1204 Add firebase object and capability

fcm/lib/converter/firebase_registration_id_nullable_converter.dart

@@ -0,0 +1,13 @@
+import 'package:fcm/model/firebase_registration_id.dart';
+import 'package:jmap_dart_client/jmap/core/id.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+class FirebaseRegistrationIdNullableConverter implements JsonConverter<FirebaseRegistrationId?, String?> {
+  const FirebaseRegistrationIdNullableConverter();
+
+  @override
+  FirebaseRegistrationId? fromJson(String? json) => json != null ? FirebaseRegistrationId(Id(json)) : null;
+
+  @override
+  String? toJson(FirebaseRegistrationId? object) => object?.id.value;
+}