Provide a way to fluently convert an response with WebClient and add BlockingWebClient

[ikhoon]

Motivation:

• Improved usability
  • REST API uses a unary call(non-streaming request and response) to send
    and respond to a message.
    As WebClient supports streaming requests and responses by default,
    users need to manually aggregate the returned HttpResponse.
  • Testing code needs to wait for the result with .join() or .get()
    to assert the result.
    If we provide BlockingWebClient, users and dev can remove a lot of
    boilerplate code.
• Possibility of performance optimization
  • If a request and response type are known before sending a call, we
    can optimize the performance for non-streaming calls.
  • ExchangeType introduced in Provide a way to enable/disable streaming for a request or response #3956 will be also used at the client side.

Modifications:

• Add ResponseAs<T, U> that defines a way to convert a response into another.
  • Decoders for byte[], String, JSON, File are provided by default.
• Add TransformingRequestPreparation<T, R> that fluently transforms a returns response.
• Add FutureTransformingRequestPreparation that is a specialized converter for CompletableFuture.
  • Scala's Future will be supported via ScalaResponseAs.
• Add BlockingWebClient that returns AggregatedHttpResponse by default.
• Add InvalidHttpResponseException for notifying a failure of JSON decoding.
• Introduce {Request,Response}Entity<T> for express a decoded type of an HTTP content.
• Migrate some tests code for proof of concept.

Result:

• You can now fluently convert a response using WebClient.

  WebClient client = WebClient.of("api.example.com");
  CompletableFuture<ResponseEntity<MyObject>> response =
     client.prepare()
          .get("/v1/items/1")
          .asJson(MyObject.class)
          .execute();

• You can now use BlockingWebClient to wait for a response to be completed.

  BlockingWebClient client = BlockingWebClient.of("https://api.example.com");
  ResponseEntity<MyObject> response =
      client.prepare()
            .get("/v1/items/1")
            .asJson(MyObject.class)
            .execute();

[codecov]

Codecov Report

Merging #4021 (f7dece4) into master (9e5c65a) will decrease coverage by 0.26%.
The diff coverage is 38.68%.

@@             Coverage Diff              @@
##             master    #4021      +/-   ##
============================================
- Coverage     73.40%   73.13%   -0.27%     
- Complexity    15931    16056     +125     
============================================
  Files          1386     1404      +18     
  Lines         60869    61433     +564     
  Branches       7715     7731      +16     
============================================
+ Hits          44680    44929     +249     
- Misses        12278    12585     +307     
- Partials       3911     3919       +8     

Impacted Files	Coverage Δ
...inecorp/armeria/internal/consul/CatalogClient.java	0.00% <0.00%> (ø)
...main/java/com/linecorp/armeria/client/Clients.java	65.27% <ø> (ø)
...spring/web/reactive/ArmeriaClientConfigurator.java	100.00% <ø> (ø)
...pring/web/reactive/ArmeriaClientHttpConnector.java	79.31% <ø> (ø)
...armeria/client/TransformingRequestPreparation.java	6.25% <6.25%> (ø)
...ia/client/BlockingWebClientRequestPreparation.java	15.55% <15.55%> (ø)
...a/client/FutureTransformingRequestPreparation.java	28.07% <28.07%> (ø)
...inecorp/armeria/common/NoHttpContentException.java	28.57% <28.57%> (ø)
...com/linecorp/armeria/client/BlockingWebClient.java	29.62% <29.62%> (ø)
.../com/linecorp/armeria/client/DefaultWebClient.java	87.71% <33.33%> (-3.19%)	⬇️
... and 47 more

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[trustin]

LGTM once the build passes. Great work, @ikhoon 🙇

[ikhoon]

LGTM once the build passes.

Oops... 😅

[minwoox]

Mostly nits.
Thanks and great job, @ikhoon!

[jrhee17]

Looks good! Left some minor nits and questions 🙏

[jrhee17]

Can you check the scala test compilation failure? 😅

[ikhoon]

Can you check the scala test compilation failure? 😅

😱😱😱

[jrhee17]

Thank you @ikhoon ! Looking forward to reviewing the exchangeType PR as well 🙇 👍 🥇