Implement socks5 support

This adds following settings:

- Socks5Enabled
- Socks5Host
- Socks5Port
- Socks5User
- Socks5Password

Currently http requests and dns requests are not getting executed as they currently can't get tunneled through socks5 proxy. Therefore gmail with oauth2 wont work through tor.

src/imap/session.rs

@@ -3,6 +3,7 @@ use std::ops::{Deref, DerefMut};
 use async_imap::Session as ImapSession;
 use async_native_tls::TlsStream;
 use async_std::net::TcpStream;
+use fast_socks5::client::Socks5Stream;
 
 #[derive(Debug)]
 pub(crate) struct Session {
@@ -17,6 +18,7 @@ pub(crate) trait SessionStream:
 impl SessionStream for TlsStream<Box<dyn SessionStream>> {}
 impl SessionStream for TlsStream<TcpStream> {}
 impl SessionStream for TcpStream {}
+impl SessionStream for Socks5Stream<TcpStream> {}
 
 impl Deref for Session {
     type Target = ImapSession<Box<dyn SessionStream>>;