Support for quiver's Optional

[sir-boformer]

It would be nice if there was a simple way to serialize and deserialize properties wrapped in an Optional:
https://github.com/google/quiver-dart/blob/master/lib/src/core/optional.dart

[kevmoo]

You could create your own TypeHelper or use the fromJson/toJson annotations on a field.

I don't want to add a dependency on quiver to this package, although it'd be easy to create a supplementary package to support those.