TF-4169 Create Labels capability and Label object

labels/lib/converter/hex_color_nullable_converter.dart

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