`hasPermittedSubclasses` and `hasNoPermittedSubclasses` for `Class` assertions

We should add `hasPermittedSubclasses(Class<?>...)` and `hasNoPermittedSubclasses()` to `Class` assertions, following [`Class::getPermittedSubclasses`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Class.html#getPermittedSubclasses()).

As we still target Java 8, the implementation should be reflection-based, similar to #3080.