[http-client] Add @Headers Annotation

[SentryMan]

Adds something like Retrofits @Headers (I got sick of making default methods to add preset headers)

Now we can do stuff like this:

@Client
@Headers("Content-Type: applicaton/json")
public interface TitanFall {

  @Get("/${titan}/${drop.point}")
  @Headers("Something: \\invalid\n\t")
  Titan titanFall();


  @Get("/${titan}/copium")
  @Headers("      Accept    :   applicaton/json")
  Titan titanFall3();
}

and the generated class will have

  // GET /${titan}/${drop.point}
  private static final String DROP_POINT = System.getProperty("drop.point");
  private static final String TITAN = System.getProperty("titan");
  @Override
  public Titan titanFall() {
    return client.request()
      .header("Something", "\\invalid")
      .header("Content-Type", "applicaton/json")
      .path(TITAN).path(DROP_POINT)
      .GET()
      .bean(Titan.class);
  }

  // GET /${titan}/copium
  @Override
  public Titan titanFall3() {
    return client.request()
      .header("Accept", "applicaton/json")
      .header("Content-Type", "applicaton/json")
      .path(TITAN).path("copium")
      .GET()
      .bean(Titan.class);
  }