[Serializer] Serialize and deserialize from abstract classes

[javiereguiluz]

This new serializer feature allows to serialize and de-serialize abstract classes. Such feature is especially useful when dealing with domain objects. See symfony/symfony#24375.

@sroze the description you added in your pull request is very easy to understand. Maybe you could reuse it or expand it to add some docs about this feature that you added to Symfony? Thanks!