Add extractor for remote connection info

[davidpdrsn]

Fixes #43

With this you can get the remote address like so:

use axum::{prelude::*, extract::ConnectInfo};
use std::net::SocketAddr;

let app = route("/", get(handler));

async fn handler(ConnectInfo(addr): ConnectInfo<SocketAddr>) -> String {
    format!("Hello {}", addr)
}

// Starting the app with `into_make_service_with_connect_info` is required
// for `ConnectInfo` to work.
let make_svc = app.into_make_service_with_connect_info::<SocketAddr, _>();

hyper::Server::bind(&"0.0.0.0:3000".parse().unwrap())
    .serve(make_svc)
    .await
    .expect("server failed");

This API is fully generic and supports whatever transport layer you're using with Hyper. I've updated the unix domain socket example to extract peer_creds and peer_addr.

[Roguelazer]

I've only had time to give this a cursory glance, but it looks great! Thanks for looking into this so fast!