Add initial SSE support

[yoshuawuyts]

This adds initial support for Server-Sent Events. It provides both a pre-made endpoint handler, and an upgrade function for use inside manual endpoints. This is the first step towards more elaborate SSE handling. Closes #234.

Future Directions

As described in my tide channels post there are many more things we could do around channels. But those rely on having a notion of identity for connecting clients, which in turn relies on session support. Once we have that we can add persistent connection IDs and more!

Additionally we're not sending automated hearbeat messages yet, I believe we're still missing support for these in http-rs/async-sse.

Screenshots

[jbr]

I'm curious about what this would look like in a use case where the events are triggered elsewhere in an application (which I assume is the vast majority of the reason to use SSE). If each connection gets a new async-sse sender/encoder pair, is the idea that within the sse:endpoint fn, users would do something like this?

app.at("/sse").get(sse::endpoint(|req: Request<SomeSharedEventStream>, sender: Sender| async move {
  while let Some(event) = req.state().recv().await {
    sender.send(event.name, event.data_as_u8_slice(), event.id).await;
  }
  Ok(())
}))

[yoshuawuyts]

@jbr as I wrote in my post on Tide Streams we can provide high-level APIs for this, but we first need a notion of identity so we can reliably match incoming requests to existing connections. But what you shared so far could indeed be a way to attempt at building something like that before we fully support it.

[goto-bus-stop]

I think the id argument to encoder.send() should prob not be hardcoded to None. You need that if you want to implement reconnections correctly without dropping messages, and a future session-based API would likely need to do just that.

In that vein, it might be handy to store the "Last-Event-ID" header value on the sender object, so you can access it easily:

sse::upgrade(req, |req, sse| async move {
  if let Some(event_id /*: &str */) = sse.last_event_id() {
    // handle that
  }
})

That could be a later addition since you can also find it on the req object already.

[yoshuawuyts]

@goto-bus-stop good call! -- I've changed Sender so it now takes an optional third argument of id, matching the API in async-sse.