DRAFT: add HTTP proxy support

[juxtin]

Users reasonably expect that the HTTP_PROXY and HTTPS_PROXY environment variables will be respected by dependency submission actions that rely on this library. However, those users are sadly disappointed for now.

This is little more than some notes on how to proceed here:

• if HTTP_PROXY or HTTPS_PROXY is set, we'll use that to create a proxyAgent
• we'll need to pass that proxyAgent as the request.agent param to octokit.request
• the tests can set up a fake proxy and use it to verify that the settings are being honored