Provide a way to automatically deserialize non-OK JSON response using `WebClient` and `RestClient`

WebClient or RestClient is able to deserialize a JSON response into an object using:

WebClient client = WebClient.of(...);
ComletableFuture<ResponseEntity<MyItems>> response = 
  client.prepare()
        .get("/api/items/...")
        .asJson(MyItems.class)
        .execute();

The client tries to convert the JSON response into an object when the class of response status is a success. https://github.com/line/armeria/blob/ccf630c417bc2f9f78a90bad53a999cc7aa46314/core/src/main/java/com/linecorp/armeria/client/AggregatedResponseAs.java#L58-L62 I believe that most REST clients expect a 2xx status for normal situations. However, when it comes to tests, it would be inconvenient. Users might want to test an error response.

So it could be useful to add some variant methods of .asJson(...) of WebClient and .execute(...) of RestClient

ComletableFuture<ResponseEntity<MyError>> response = 
  client.prepare()
        .get("/api/items/...?bad=parameter")
        .asJson(MyError.class, HttpStatus.INTERNAL_SERVER_ERROR)
        .execute();

In addition to HttpStatus, we can also add Predicate<HttpStatus> and HttpStatusClass as a parameter.

[ikhoon]

Thanks for the pull request. I will take a look at it soon.

[ikhoon]

We can implement HttpStatusPredicate like the following.

final class HttpStatusPredicate extends Predicate<HttpStatus> {

    @Nullable
    private HttpStatus status;
    @Nullable
    private HttpStatusClass statusClass;
     
    ...    

    @Override
    boolean test(HttpStatus status) {
        if (this.status != null) {
            return this.status.equals(status);
        }
        assert statusClass != null;
        return status.codeClass() == statusClass;
    }

    @Nullable
    HttpStatus status() {
        return status;
    }

    @Nullable
    HttpStatusClass statusClass() {
        return statusClass;
    }
}

The custom HttpStatusPredicate will carry the desired status information, and we can refactor AggregatedResponseAs to use HttpStatusPredicate for a simple condition.

// In AggregatedResponseAs
private static final HttpStatusPredicate SUCCESS_PREDICATE = new HttpStatusPredicate(HttpStatusClass.SUCCESS);

private static <T> ResponseEntity<T> 
    newJsonResponseEntity(AggregatedHttpResponse response, 
                          JsonDecoder<T> decoder) {
     return newJsonResponseEntity(response, decoder, SUCCESS_PREDICATE);
}

private static <T> ResponseEntity<T> 
    newJsonResponseEntity(AggregatedHttpResponse response, 
                          JsonDecoder<T> decoder, HttpStatus status) {
     return newJsonResponseEntity(response, decoder, new HttpStatusPredicate(status));
}

private static <T> ResponseEntity<T> 
    newJsonResponseEntity(AggregatedHttpResponse response, 
                          JsonDecoder<T> decoder, Predicate<HttpStatus> status) {
    if (status instanceof HttpStatusPredicate) {
       ... // Extract either HttpStatus or HttpStatusClass to provide the expected status.
    }
}