New protocol: SOCKS5

[fortuna]

Version 0.0.4 adds support for SOCKS5 StreamDialers, which allows you to use SOCKS5 servers to proxy your connections.

Unlike traditional implementations, ours use a single roundtrip for the handshake.

Documentation is at https://pkg.go.dev/github.com/Jigsaw-Code/outline-sdk/transport/socks5

You can find usage examples in the test file.

We don't yet support UDP, but we plan to eventually.

[fortuna]

FYI, I was not checking the server reply for errors before, but that was fixed in #56. It should be available in v0.0.5.