Add three settings:
- http.proxy-auth
- http.proxy-username
- http.proxy-password

proxy-auth defaults to "auto" so that most use cases work out of the
box. I don't know why you would disable authentication support, but I
added a "disable" option in this case. You can also force the use of a
mechanism and abort if the proxy doesn't advertise it.

proxy-username and proxy-password: String vs Option<String>?
curl doesn't negotiate (SPNEGO, here "gss") if handle.proxy_username and
handle.proxy_password aren't called. This authentication mechanism
doesn't require a user/pass in cargo config. This shouldn't hurt (?)
when the proxy doesn't require authentication.

About curl-rust's spnego and ntlm features:
https://github.com/alexcrichton/curl-rust/blob/b50c5d355aab69483752db51815c5a679b29d6c4/curl-sys/Cargo.toml#L54-L60
**This is the last blocker I think.** If we require those features, then
vendored curl builds will require "a suitable GSS-API library or SSPI on
Windows" [0] for the "gss" setting to work (which is what corporate
proxies are using most of the time). I think this PR is blocked until we
can either  vendor those dependencies, or we change the official cargo
builds to link dynamically to libcurl.

this is true! my bad!

<rust-lang#11433 (comment)>

handle.proxy_username and handle.proxy_password still need to be called
when options are "None", otherwise curl won't negotiate in cases it
should (eg when using "gss")

<rust-lang#11433 (comment)>