Provide a way to send a JSON object with `WebClient`

JSON is the dominant exchange format for REST API. However, Armeria's WebClient does not provide any JSON-specific APIs. I believe it should be useful additions if we provide:

WebClient client = WebClient.of();
// Send a serialized JSON object with "applicaiton/json"
HttpResponse response = client.postJson("/items", new MyItem()); 
HttpResponse response = client.putJson("/items", new MyItem());
HttpResponse response = client.patchJson("/items", new MyItem());

HttpResponse response = client.prepare()
                              .post("/items")
                              .contentJson(new MyItem())
                              .execute();