refactor: one blocking SOCKS5/HTTP CONNECT proxy module shared by all TCP clients

SSH's async SOCKS5 and HTTP CONNECT code moves to clients::proxy as a
blocking dialer (SSH runs it on a blocking thread), so RDP and others can
use the same implementation. The proxy handshake now has a read timeout,
and new tests drive both proxy types against fake proxies, including
SOCKS5 username/password auth and a refused HTTP CONNECT (407).

rust/core/src/clients/mod.rs

@@ -37,6 +37,7 @@ impl Callbacks {
     }
 }
 
+pub mod proxy;
 pub mod raw;
 pub mod serial;
 pub mod telnet;