Provide a way to get `ClientRequestContext` without using a decorator.

Currently, a user has to insert a decorator to access the ClientRqeustContext or its RequestLog. For example:

HttpClient client = new HttpClientBuilder(...)
    .decorator((delegate, ctx, req) -> {
        // Access ctx and ctx.log()
    })
    .build();

It would be pretty convenient if a user can do this:

HttpClient client = HttpClient.of(...);
HttpResponse res = client.get("/");
ClientRequestContext ctx = ClientRequestContext.get(res);

[minwoox]

Had a chat with @trustin and we have to consider RpcResponse as well. Also, there’s a chance that the client is wrapped by another layer, so I think this is a very difficult issue to solve. 😢 We need to think about it a little bit more. 😅

[minwoox]

Because any implementations of an HttpResponse can be returned using a decorator, I think there are two ways to solve this:

• Let the HttpResponse have the ClientRequestContext if it’s in the thread-local when the response is created (I think this is not a good choice)
• Wrap the returned HttpResponse somewhere (perhaps in UserClient?) if it does not have the ClientRequestContext

  // Do not have to wrap this.
  class DecodedHttpResponse extends ... implements RequestContextHolder {...}
  // Other cases.
  class HttpResponseWithClientRequestContext implements HttpResponse, RequestContextHolder {...}
  
  // And getter
  
  public interface ClientRequestContext {
      @Nullable
      ClientRequestContext get(HttpResponse res) {
          if (res instance of RequestContextHolder) {...}
          ...
      }
  }
  

What do you think?