Specify schema processing for sealed classes and permitted subclasses

[MikeEdgar]

Sealed classes/interfaces in Java 17+ allow for one or more permitted subclasses to be specified on the sealed class. This seems to be a good match for JSON schemas anyOf subschema array. We should investigate if this is a good feature for inclusion in MP OpenAPI.

[Azquelt]

I'm not sure whether we can specify anything for this without being a lot more specific about how schemas get generated.

It's certainly something that implementations could do, but without having spec requirements I'm not sure what we could require in the TCK.

[Azquelt]

Removing this from the milestone until we can depend on Java 17.