[REQ] Java: generate sealed interfaces for oneOf

[inaldt]

Is your feature request related to a problem? Please describe.

The current handling of oneOf does not allow switch expressions on the generated type to be exhaustive.

Currently oneOf generates an interface:

Animal:
  oneOf:
    - $ref: '#/components/schemas/Cat'
    - $ref: '#/components/schemas/Dog'

generates

public interface Animal {...}
public class Cat implements Animal {...}
public class Dog implements Animal {...}

Switching over this requires a default clause:

switch (myAnimal) {
  case Dog d -> "dog"
  case Cat c -> "cat"
  default -> "There is no other class that implements Animal but I still need this clause here"
}

Avoiding default clauses in switch expressions is considered good practice as it allows the compiler to check for omissions.

Describe the solution you'd like

Animal:
  oneOf:
    - $ref: '#/components/schemas/Cat'
    - $ref: '#/components/schemas/Dog'

generates

public sealed interface Animal permits Cat, Dog {...}
public class Cat implements Animal {...}
public class Dog implements Animal {...}

Describe alternatives you've considered

The suggested feature is the canonical way to express sum types in Java, hence the canonical fit for oneOf.

Additional context