Support subscriptions using SSE (Server-Sent Events) as alternative to websockets

[cmeeren]

I propose that support is added for the GraphQL over Server-Sent Events Protocol, like HotChocolate has.

This simplifies authentication, since you can use the same authentication as normal HTTP requests.

I haven't looked at the GraphQL-specific parts of it, but SSE it itself technically very simple. Here is a demo:

Get(
 "/sse-test",
 RequestDelegate(fun (ctx: HttpContext) ->
 task {
 ctx.Response.Headers.Add("Content-Type", "text/event-stream")

 while not ctx.RequestAborted.IsCancellationRequested do
 do!
 ctx.Response.WriteAsync(
 $"data: {DateTime.Now}"
 )

 do! ctx.Response.WriteAsync("\n\n")
 do! ctx.Response.Body.FlushAsync()
 do! Async.Sleep(1000)
 }
 )
)

You can test it by simply opening that endpoint in a browser (at least Edge works fine).

Related: #460

[xperiandri]

Good idea, why not!