Add support for SOCKS5 state machine

[sethmlarson]

We shouldn't be able to make requests before we're authenticated successfully.

[yeraydiazdiaz]

I came across https://github.com/glyph/automat some time ago, would it be a good fit?

It might be overkill if the state machine is simple though since h11 implemented its own.

[sethmlarson]

Eh, let's implement our own state machine, I think there's only a few states for TCP:

CLIENT_AUTH_REQUIRED
SERVER_AUTH_REPLY
CLIENT_AUTHENTICATED
TUNNEL_READY

Let's keep the states identical for SOCKS4 and SOCKS5 cuz 4 doesn't really have a state machine. :)

We'll have to figure out how to handle SOCKS5 UDP for HTTP/3.

[yeraydiazdiaz]

I believe this is covered now?

We'll have to figure out how to handle SOCKS5 UDP for HTTP/3.

UDP is addressed separately in #4

[sethmlarson]

Forgot to close this!