websockets: configure default ping interval

[oliver-sanders]

Websocket servers can send pings to the client, if the client does not respond within a configured timeout, the server can close the connection.

This helps to identify client disconnects before failed data delivery which can be useful. This can also help to keep the connection looking "alive" from the perspective of proxies, some of which will kill websockets on an idle timeout.

The ability to configure pings was recently added to Jupyter Server (#1391), once this is released we will be able to configure websocket pings.

Suggest being a little generous with the timeout as we know that some blocking requests can take several seconds to be processed (or over 10 seconds in some cases, see #547).