Add automatic ping handling / timeouts to web sockets

[Kaliumhexacyanoferrat]

As an operator of a webserver having websocket connections, I would like inactive connections to be dropped, so that resources are freed and my server cannot be attacked using a denial of service attack.

Example

Websocket.Reactive().InactivityTimeout(Timeout.FromSeconds(60));

Acceptance criteria

• The functionality can be easily enabled for functional and reactive handlers
• If possible, the functionality does not rely on additional threads being spawned
• The feature is documented on the GenHTTP website
• The feature is covered by acceptance tests