feat: Add --no-proxy-env and support --proxy="" (#13051)

Either option will disable pip from reading proxy configuration from
http_proxy, https_proxy, or no_proxy. We're adding two knobs because we
originally said that --proxy="" should work (which didn't... to much
confusion and frustration), but this trick can't be (easily) set via
envvar or configuration file.

(kids, consider this a lesson to be careful about making an unverified
claim in public, it may come back to haunt you nearly 8 years later...
*wink*)

Co-authored-by: Damian Shaw <damian.peter.shaw@gmail.com>
Commit-message-authored-by: Richard Si <sichard26@gmail.com>