Add HTTP/2 keep-alive PING policy.

[arturobernalg]

This introduces an HTTP/2 keep-alive PING policy for long-lived idle connections. When enabled, the client sends PING frames once the SETTINGS handshake is complete and no streams are active, and fails the session if the peer does not respond with PING[ACK] within the configured timeout. Integration tests validate that the connection remains usable past idle with keep-alive enabled and closes on inactivity when disabled.

[ok2c]

@arturobernalg The change-set will likely need some work, but I am conceptually fine with it. @rschmitt Would you have any conceptual objections to the idea?

[rschmitt]

When enabled, the client sends PING frames once the SETTINGS handshake is complete and no streams are active, and fails the session if the peer does not respond with PING[ACK] within the configured timeout.

This sounds way better than the current HTTP/2 implementation of setValidateAfterInactivity, which adds an entire round trip's worth of latency to the request path. Would it be outrageous if exposed this through setValidateAfterInactivity instead of adding something like a new H2Config option?

[ok2c]

@arturobernalg

1. I also think H2Config is not the right place. Keep it as a separate constructor parameter. Once available in client H2PingPolicy should go into ConnectionConfig
2. Any chance you could use the existing ping command here?

[rschmitt]

Once available in client H2PingPolicy should go into ConnectionConfig

I think we have too many config knobs already. I'd really prefer to expose this through setValidateAfterInactivity. Once we have support for HTTP/2 keep-alive PINGs that run asynchronously in the background, no one in their right mind will ever want to send a blocking PING in the request path to validate a connection. (I've spent this week dealing with a set of production issues that are directly related to this problem, and it has radicalized me re: how badly I want this keep-alive PING functionality.)

[arturobernalg]

keep-alive is now driven via setValidateAfterInactivity (no new H2Config knob), so it fits the existing connection validation API.
Also switched to the existing ping command path (keep-alive uses the same sendPing flow), per your suggestion.