Motivation:

`WebClient` and `WebClientPreparation` has powerful and fluent features.
RESTful API is classic but it is still the dominant protocol for sharing
web resources.
The `WebClient` is able to handle not only REST responses but also
a stream response and a file response.
The API design of `WebClient`, in turn, is not optimized for REST API.
See #4250 

Modifications:

- Add `RestClient` for exchange RESTful APIs.
  - It assume that the response has a JSON body.
- Add `RestClientBuilder` so as to fluently build a `RestClient`.
- Refactor the intefaces for request preparations to accommodate
  `RestClientPreparation`.
  - Method and path setters are extracted to `RequestMethodSetters`.
  - Query and path param setters are moved to `PathAndQueryParamSetters`
  - Added `HttpMessageSetters` for setting `HttpMessage` properties.
- Add `execute()` extension methods for Kotlin Coroutine.
- Add `ScalaRestClient` for Scala.
  - Tried to add extension methods using an implicit class but Scala
    compiler failed to find the overloaded method which has a generic
    parameter.
- Provide a extension method to convert a `WebClient` to
  `ScalaRestClient`.
- Add `restClient()` to `ServerExtension` and `ServerRule`.

Result:

You can now easily send and receive RESTful APIs using `RestClient`
- Java
  ```java
  RestClient restClient = RestClient.of("...");
  CompletableFuture<ResponseEntity<Customer>> response =
      restClient.get("/api/v1/customers/{customerId}")
                .pathParam("customerId", "0000001")
                .execute(Customer.class);
  ```
- Kotlin
  ```kt
  val restClient: RestClient = RestClient.of("...");
  val response: ResponseEntity<Customer> =
     restClient
       .get("/api/v1/customers/{customerId}")
       .pathParam("customerId", "0000001")
       .execute<Customer>()  // a suspend function
  ```
- Scala
  ```scala
  val restClient: ScalaRestClient = ScalaRestClient("...")
  val response: Future[ResponseEntity[Result]] =
    restClient.post("/api/v1/customers")
              .contentJson(new Customer(...))
              .execute[Result]()
  ```