Add command line client with SOCKS

[ha-D]

Adds a command line client for the outline-ss-server. This should help with easier prototyping of features without having to go through the whole stack.

It starts a SOCKS server, accepts TCP connections and proxies them to the configured outline server.
Non-goals for this PR (can be added later):

• UDP
• Automatically setting the system proxy

--

Two decisions I made which are worth discussing:

• Should it have a separate main or should we implement it as part of the same (server) binary and distinguish between running the server and client using a flag (similar to what go-shadowsocks2 does). I prefer separate myself and thats what I went with (though maybe we want to restructure packages a bit), let me know if you think it's better to go with a single binary.

• How should the client take in the server configurations (e.g, host, port, password)

  • Using separate flags
    go run cmd/client -p port -s server -m cipher -k password
  • Using configuration file
    go run cmd/client -c config.yaml
    This will have to be a different config format that the one used for the server since the client can't take multiple keys
  • Using a single flag to pass in key url
    go run cmd/client -k ss://[email protected]:8080
    This is the one I'm going with right now. I'm not sure if the way I parse the key is fully compilable with SIP002 but I think its compilable enough (?).

[bemasc]

This is looking good!

[fortuna]

Thanks for the contribution!

[fortuna]

This should probably be []bytes

[ha-D]

I'm going to have to string it again when passing to client.NewClient. Do you think I should still keep it []byte here?

[fortuna]

Could you create a README.md file explaining how to run the socks proxy? Mention it only supports TCP.

[ha-D]

Would you rather have a separate README in cmd/client or do you think I should update the original README to change the go-shadowsocks2 client to this one?

[ha-D]

Thanks for the quick reviews!!

[fortuna]

@ha-D I found https://github.com/things-go/go-socks5. It seems it's a lot easier to use that library, and it supports UDP too.
I think all you need to do is call NewServer with https://pkg.go.dev/github.com/things-go/go-socks5#WithDial, taking a wrapped Shadowsocks client as the Dialer.