SOCKS5 authentication support

[riastradh-brave]

In order to isolate circuits for different sessions and first-party origins, we need to specify a distinct username/password pair for each isolation domain when making requests to the tor socks proxy.

Draft is currently in progress here: https://github.com/riastradh-brave/muon/tree/riastradh-socks5-auth

[riastradh-brave]

This will require some work to rebase on top of Chromium 64, since I did the initial work on Chromium 63 before we updated to 64!