Support proxy authentication

[ptwales]

Problem to solve

Use hurl with a proxy that requires a username and password.

Proposal

Support the --proxy-header option from curl in hurl so hurl can send basic auth headers to the proxy. Or support the --proxy-basic and --proxy-user options.

Ideally hurl would also support the https://user:pass@host:port format for the --proxy option, but I think that might cause round trips to figure out which authentication schema to use.

Somewhat related to #3861