Proxy VCS HTTP requests

Summary:
Ref T7019. When we receive a git clone https:// (or git push on HTTP/S), and the repository is not local, proxy the request to the appropriate service.

This has scalability limits, but they are not more severe than the existing limits (T4369) and are about as abstracted as we can get them.

This doesn't fully work in a Phacility context because the commit hook does not know which instance it is running in, but that problem is not unique to HTTP.

Test Plan:

• Pushed and pulled a Git repo via proxy.
• Pulled a Git repo normally.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7019

Differential Revision: https://secure.phabricator.com/D11494