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:

• Kestrel does not generate a Keep-Alive header for HTTP/1.1, so we can remove this as well
• Kestrel uses lower-case for cookie properties
• Enable HTTP/2 and HTTP/3 with Kestrel #554
• Kestrel keeps multiple slashes in the url, Kestrel does not (see TestMultipleSlashes)
• Kestrel replaces + with a blank in an URL, GenHTTP does not (see TestQueryWithSpaces)
• 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