socks: Implement proposal 351.

Nick Mathewsonrequested to merge
nickm/arti:impl-prop-351 into main

See https://spec.torproject.org/proposals/351-socks-auth-extensions.html

This proposal changes the interpretation of SOCKS5 usernames/passwords to give a more principled and extensible way of getting RPC IDs and isolation strings.

This addresses the Arti side of #1615 (closed).

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