WIP: Add ClientInfo type to ListeningSocket

[jcspencer]

This PR is very much a work-in-progress, and I'm still unsure on a few things - but here goes nothing!

This adds:

• A type ClientInfo type parameter to ListeningSocket
• A fn client_info(stream: &Self::Stream) -> io::Result<Self::ClientInfo> method to ListeningSocket

The idea here being that a new Session is initialized with information about the connecting client, such as:

New connection from client: UnixClientInfo {
   socket_addr: (unnamed),
   credentials: UCred { pid: Some(73721), uid: 501, gid: 20 }
}

or on Windows:

New connection from client: NamedPipeClientInfo { pid: 2684, session_id: 1 }

---

Now, for the issues:

• Type information: I can't figure out the best way to pass type information to the Agent implementation so that it can actually extract fields from the ClientInfo (specific to the Stream type)
• Windows: Oh man, don't even get me started on Windows!
  • To get anything usable from the name pipe, we need to make two unsafe calls to get the PID and Session ID for the client...

---

Alternatively, we can let Agent implementations extract this information from sockets themselves, but this would likely require an enum specifying the socket type, so you can consume socket-specific types. Not quite sure what the best option is - would love your thoughts!

[jcspencer]

For what it's worth, you can do something like this in nightly, but it isn't exactly pleasant (though it does work!):

impl Agent for KeyStorageAgent {
    fn new_session<S>(&mut self, socket: &S::Stream) -> impl Session
    where
        S: ListeningSocket + std::fmt::Debug + Send,
    {
        if let Ok(client_info) = S::client_info(socket) {
            match &client_info as &dyn Any {
                _ if let Some(tcp) = s.downcast_ref::<TcpClientInfo>() => {
                    println!("New connection from TCP client: {:?}", tcp);
                },
                #[cfg(unix)]
                _ if let Some(unix) = s.downcast_ref::<UnixClientInfo>() => {
                    println!("New connection from Unix client: {:?}", unix);
                },
                #[cfg(windows)]
                _ if let Some(named_pipe) = s.downcast_ref::<NamedPipeClientInfo>() => {
                    println!("New connection from Named Pipe client: {:?}", named_pipe);
                },
                _ => {}
            };
            
        }

        KeyStorage {
            identities: Arc::clone(&self.identities),
        }
    }
}

[jcspencer]

Closing in favor of #60