TF-4195 Add interactor/repository/datasource to feature add a Label to an email

labels/lib/converter/keyword_identifier_nullable_converter.dart

@@ -0,0 +1,14 @@
+import 'package:jmap_dart_client/jmap/mail/email/keyword_identifier.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+class KeywordIdentifierNullableConverter
+    implements JsonConverter<KeyWordIdentifier?, String?> {
+  const KeywordIdentifierNullableConverter();
+
+  @override
+  KeyWordIdentifier? fromJson(String? json) =>
+      json != null ? KeyWordIdentifier(json) : null;
+
+  @override
+  String? toJson(KeyWordIdentifier? object) => object?.value;
+}