…d `BlockingWebClient` (#4021)

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