Add an AphrontHTTPProxyResponse

Summary:
Ref T7019. Adds a new response which can proxy an HTTP request and pass the result through.

This is grossly inefficient for the same reasons as HTTP hosting is generally inefficient right now (T4369). This stuff is fixable but not trivial.

Test Plan: Replaced home page with a proxy to example.org, used Charles to view headers, saw the page headers and content proxy with an X-Phabricator-Proxied header.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7019

Differential Revision: https://secure.phabricator.com/D11492