Fixes https://github.com/tokio-rs/axum/issues/43
With this you can get the remote address like so:
```rust
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`.
Not actually related to Axum, can be implemented directly with Hyper, but I figure its nice to have for demonstration and might help catch accidental breaking changes in the future.
