Add a reactive CommandLineRunner variant

[bclozel]

See discussion in #9761.

Spring Boot provides a functional interface CommandLineRunner for running tasks.
The method signature works well for blocking operations, but can lead to user errors if it's used with a reactive library.

I've seen a lot of code samples out there just calling Flux.subscribe(Consumer) in a CommandLineRunner.run call — this won't guarantee that the processing of that Flux is done before the application is shut down/the webapp is started.

We could create something like:

@FunctionalInterface
public interface ReactiveCommandLineRunner {
  Mono<Void> run(String... args);
}

We could run all of those in parallel, or, at processing time, we could concat all of them (to guarantee a processing order) and block, like:

firstMono.concatWith(secondMono).then().block();

[bclozel]

Declining this for now - this is more a usability issue about Reactive types.