Add java.net.http.HttpClient based snippet generator

[wtetsu]

Hello, I added Java code generator that uses java.net.http.HttpClient.

There are already some Java implementations,
but it seems all of them use third party library that are need to be installed.

Advantage:
This code is based on the Java standard library so it's available without installing any third-party library in Java 9+.
and code is concise enough, I think.

Basic code style:

HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("http://mockbin.com/har"))
    .header("content-type", "text/plain")
    .method("POST", HttpRequest.BodyPublishers.ofString("Hello World"))
    .build();
HttpResponse<String> response = HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(response.body());