Make http_server part of the sdk.

mojo/services/http_server/public/interfaces/http_response.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module http_server;
 
 struct HttpResponse {
   // The HTTP status code.
   uint32 status_code = 200;
 
   // The payload for the response body.
   handle<data_pipe_consumer>? body;
 
   // This must be the number of bytes in |body|.
   int64 content_length = 0;
 
   // The content type.
   string content_type = "text/html; charset=utf-8";
 
   // Additional HTTP response headers.
   map<string, string>? custom_headers;
 };