Provide proxy service out of box

[yzfeng2020]

Jetty offers a built-in solution for forwarding requests to another server through its AbstractProxyServlet and related subclasses. It would be beneficial for Armeria to provide a similar feature. While Armeria has ProxyService, it is not fully production-ready.

The following extension points that I can think of for this feature are:

• Client provision (e.g. the configuration of the Web client);
• Modification of incoming requests (e.g. rewriting the url, add / modify request headers);
• Mapping of outgoing responses (e.g. remap the status code, modify response headers and/or change the response content).

Opening this issue to start the discussion on the design of the interfaces for this feature, as the ones provided by Jetty seem complex in my opinion.

[misselvexu]

/Submit a PR , assigned ?

[ikhoon]

Are you interested in sending a PR?