Improve the usability of `WebClient`

From an internal feed back:

What would the ideal API for building an HTTP request look like? Do you have some imaginary ideal example?

Using method chains like Spring WebClient is easy for me to write http request. Annotation pattern like Retrofit is also easy for me.I prefer the Spring WebClient way to annotation pattern.

What is the library/API you think Armeria team should benchmark?

Spring WebClient

What other features would you like to see in Armeria?

Prepare a method which maps HttpResponse to Java Object, like bodyToMono<T>

[trustin]

Yeah, having a class like WebClientRequestBuilder would be nice. We could extract the common methods in HttpRequestBuilder into some common interface or abstract class. What do you think, @tumile ?

[trustin]

https://stackoverflow.com/questions/63660975/armeria-http-client-how-to-add-query-string-parameters#comment112614741_63664409

It may be useful if we can build/execute an HttpRequest fluently, so that, for example, add a query parameter easily.

// Not the final form - just an idea
HttpRequest req =
    HttpRequest.builder(HttpMethod.GET, "/foo")
               .queryParam("bar", "2")
               .cookie(...)
               .build();
webClient.execute(req);

// and/or:
webClient.prepare(HttpMethod.GET, "/foo")
         .queryParam("bar", "2")
         .cookie(...)
         .execute(); // or .execute(String.class) for automatic conversion