TF-1204 Add firebase object and capability

fcm/lib/converter/device_client_id_nullable_converter.dart

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