I propose adding a reactive version of UserDetailsChecker along with an implementing class that mirrors the AccountStatusUserDetailsChecker.

This is frequently used when combined with a UserDetailsService for checking the status of a user's account and we need this on the reactive side as well.

It also needs to be tied into the ReactiveAuthenticationManager implementations (i.e. UserDetailsRepositoryReactiveAuthenticationManager). I would say we open a separate ticket for that integration and we keep this ticket just for the introduction of the ReactiveUserDetailsChecker.

I'm happy to supply a PR for this as I already have it built and could submit it pretty quickly.