[http-client] Add code generation for the as() methods returning HttpResponse<E> etc

[rbygrave]

Supports code generation for client API methods like the following:

@Client
public interface MyApi {

  @Get("/as/{id}")
  HttpResponse<Repo> getAs(UUID id);

  @Get("/as-list/{id}")
  HttpResponse<List<Repo>> getAsList(UUID id);

  @Get("/as-stream/{id}")
  HttpResponse<Stream<Repo>> getAsStream(UUID id);

  @Get("/call-as/{id}")
  HttpCall<HttpResponse<Repo>> getCallAs(UUID id);

  @Get("/call-as-list/{id}")
  HttpCall<HttpResponse<List<Repo>>> getCallAsList(UUID id);

  @Get("/call-as-stream/{id}")
  HttpCall<HttpResponse<Stream<Repo>>> getCallAsStream(UUID id);
}