Known issues with the Kestrel engine integration

[Kaliumhexacyanoferrat]

This issue allows us to track the issues that still exist with the Kestrel server engine integration (as they were found in #526):

• Kestrel does not allow to set a custom HTTP status
• Kestrel does not allow to read the request body twice
• Kestrel does not allow to read the length of the request body
• Websockets are not supported as the underlying socket is not exposed by Kestrel

Additionally, the following differences were found:

• Do not send a Keep-Alive header for HTTP/1.1 #701
• Kestrel uses lower-case for cookie properties -> We will stick to this
• Enable HTTP/2 and HTTP/3 with Kestrel #554
• GenHTTP keeps multiple slashes in the url, Kestrel does not (see TestMultipleSlashes)
• Kestrel replaces + with a blank in an URL, GenHTTP does not (see TestQueryWithSpaces) -> This is a browser thing - as we intend to have API clients, we will stick to this
• We need to allow to set path and secureOnly for cookies (already adressed as an issue)
• GenHTTP sends Content-Length: 0 for an empty response, Kestrel does not -> As this is more explicit, we will keep this

[Kaliumhexacyanoferrat]

Need to re-evaluate websocket support as we switched to streams, so that might work now - just need to ensure that we do not write anything to the output connection.

app.Map("/custom-upgrade", async context =>
{
    var upgradeFeature = context.Features.Get<IHttpUpgradeFeature>();

    if (upgradeFeature != null)
    {
        var stream = await upgradeFeature.UpgradeAsync(); // This hijacks the connection!

        var writer = new StreamWriter(stream) { AutoFlush = true };
        var reader = new StreamReader(stream);

        await writer.WriteLineAsync("Hello, raw connection!");
        var line = await reader.ReadLineAsync();

        Console.WriteLine($"Received: {line}");
    }
    else
    {
        context.Response.StatusCode = 426; // Upgrade Required
    }
});