HTTP CONNECT handshake in connect_via_proxy() per RFC 7231, with optional Proxy-Authorization: Basic per RFC 7617
connect_via_proxy()
[channel] outbound_proxy = "http://127.0.0.1:8080" # or with Basic auth: outbound_proxy = "http://user:pass@192.168.1.1:3128"
will add tests after the approach is affirmed
closes: #2359