Add support for async method calls

[rbygrave]

.async().asDiscarding()

clientContext.request()
   .path("hello/world")
   .GET()
   .async().asDiscarding()
   .whenComplete((hres, throwable) -> {

     if (throwable != null) {
       ...
     } else {
       int statusCode = hres.statusCode();
       ...
     }
   });

.async().asString()

clientContext.request()
   .path("hello/world")
   .GET()
   .async().asString()
   .whenComplete((hres, throwable) -> {

     if (throwable != null) {
       ...
     } else {
       int statusCode = hres.statusCode();
       String body = hres.body();
       ...
     }
   });

.async().bean(HelloDto.class)

clientContext.request()
   ...
   .POST().async()
   .bean(HelloDto.class)
   .whenComplete((helloDto, throwable) -> {

     if (throwable != null) {
       HttpException httpException = (HttpException) throwable.getCause();
       int statusCode = httpException.getStatusCode();

       // maybe convert json error response body to a bean (using Jackson/Gson)
       MyErrorBean errorResponse = httpException.bean(MyErrorBean.class);
       ..

     } else {
       // use helloDto
       ...
     }
   });

.async().withHandler(...)

CompletableFuture<HttpResponse<Void>> future = clientContext.request()
   .path("hello/lineStream")
   .GET().async()
   .withHandler(HttpResponse.BodyHandlers.fromLineSubscriber(new Flow.Subscriber<>() {

     @Override
     public void onSubscribe(Flow.Subscription subscription) {
       subscription.request(Long.MAX_VALUE);
     }
     @Override
     public void onNext(String item) {
       ...
     }
     @Override
     public void onError(Throwable throwable) {
       ...
     }
     @Override
     public void onComplete() {
       ...
     }
   }))
   .whenComplete((hres, throwable) -> {
     int statusCode = hres.statusCode();
     ...
   });