Support keep-alive pings in `GraphQlWebSocketHandler`

[rstoyanchev]

Expose an additional input into GraphQlWebSocketHandler, probably Duration, that would enable periodic pings from the server to the client. This would allow detecting a lost connection or disconnected client, independent of how frequently messages are sent.

Originally requested under #528.

[brandonbyskov]

@rstoyanchev, It would be useful to know the keep-alive duration and timeouts when we configure other parts of our system, such as the load balancer's idle timeout. It would be useful to be able to either calculate or configure the maximum length of time we could expect a websocket connection to stay idle before being closed. It seems like there would be 2 relevant durations:

• Maximum length of an idle period before a server keep-alive ping is sent to client. Whether the ping is sent at a regular time interval or some time from last know activity.
• Timeout duration that the server would wait for a client response before closing the websocket.

[rstoyanchev]

@brandonbyskov, good point that we can skip PINGs when the session is already sending messages. For the idle read and write timeout, I was thinking that we could be more generous with read timeouts to begin with, e.g. 2 or 3 times the keep-alive period, in which case a single keep-alive duration should do. Do you agree?

[brandonbyskov]

@rstoyanchev That would be good. As long as the timeout or how it's calculated is documented somewhere, so that we can calculate the maximum potential idle period.