Support default methods in @HttpExchange interface

[wonwoo]

@HttpExchange("http://localhost:8080")
public interface GreetingClient {

    @GetExchange("/greeting")
    Flux<Greeting> greetings();

    @PostExchange("/greeting")
    Mono<Greeting> greetings(@RequestBody Greeting greeting);

    default Flux<Greeting> defaultMethod() {
        return greetings(new Greeting("default method test"))
                .flatMapMany(it -> greetings());
    }
}

defaultMethod method I hope it works!