bug#62598: 29.0.60; url-https-proxy-connect doesn't support multi-stage auth to proxies

[J.P.]

Hi, just wondering if you might be interested in broadening the scope of
this bug into something more ambitious, namely, making proxy handling
more flexible and predictable for libraries doing business with `url'.
I've been tinkering with

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=53941

off and on for a bit, but I'm not familiar enough with the `url'
landscape to go all in. From your bug description, you seem to have a
good handle on the `url-http' parts, so perhaps you're open to exploring
ideas for improving the overall proxy situation `url'-wide. If so, I'd
be willing to investigate how best to adapt `socks' to whatever you
might propose. Just a thought, though (no pressure).