TF-1204 Add firebase object and capability

fcm/lib/converter/firebase_token_nullable_converter.dart

@@ -0,0 +1,12 @@
+import 'package:fcm/model/firebase_token.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+class FirebaseTokenNullableConverter implements JsonConverter<FirebaseToken?, String?> {
+  const FirebaseTokenNullableConverter();
+
+  @override
+  FirebaseToken? fromJson(String? json) => json != null ? FirebaseToken(json) : null;
+
+  @override
+  String? toJson(FirebaseToken? object) => object?.value;
+}