Automatically create subclass serializers for sealed classes/interfaces

[Kroppeb]

Would this be default or would it require an annotation on the class? Could be default on if the base class is abstract / is an interface, but this could conflict with #14, #15 or custom serializers

[alphaqu]

This needs to be an annotation as having it default might create confusion. I think one of our rules is, don't do shit without confirmation.

[Kroppeb]

sealed classes/interfaces are still a preview feature in java 16, this might make it harder to implement/test