Server-Sent Events (#75)

Example usage:

```rust
use axum::{prelude::*, sse::{sse, Event, KeepAlive}};
use tokio_stream::StreamExt as _;
use futures::stream::{self, Stream};
use std::{
    time::Duration,
    convert::Infallible,
};

let app = route("/sse", sse(make_stream).keep_alive(KeepAlive::default()));

async fn make_stream(
    // you can also put extractors here
) -> Result<impl Stream<Item = Result<Event, Infallible>>, Infallible> {
    // A `Stream` that repeats an event every second
    let stream = stream::repeat_with(|| Event::default().data("hi!"))
        .map(Ok)
        .throttle(Duration::from_secs(1));

    Ok(stream)
}
```

Implementation is based on [warp's](https://github.com/seanmonstar/warp/blob/master/src/filters/sse.rs)

Cargo.toml

@@ -23,7 +23,7 @@ bytes = "1.0"
 futures-util = "0.3"
 http = "0.2"
 http-body = "0.4"
-hyper = { version = "0.14", features = ["server", "tcp", "http1"] }
+hyper = { version = "0.14", features = ["server", "tcp", "http1", "stream"] }
 pin-project = "1.0"
 regex = "1.5"
 serde = "1.0"
@@ -57,6 +57,7 @@ tracing = "0.1"
 tracing-subscriber = "0.2"
 uuid = { version = "0.8", features = ["serde", "v4"] }
 async-session = "3.0.0"
+tokio-stream = "0.1.7"
 
 [dev-dependencies.tower]
 version = "0.4"