Add gun:ping/2,3 for user-initiated ping for HTTP/2

[zuiderkwast]

This feature allows a user to explicitly send a ping frame and receive a ping ack. The use case is to test the latency and health of existing long-lived API connections using some admin tool.

A request, such as OPTIONS or HEAD, can also be used for this purpose, but it may be handled differently by middleware.

This PR implements it only for HTTP/2, which is the use case we have. For other protocols, an error tuple is sent to the reply-to process.

This use case is mentioned in RFC 7540, section 6.7:

The PING frame (type=0x6) is a mechanism for measuring a minimal
round-trip time from the sender, as well as determining whether an
idle connection is still functional.

Section 8.1.4:

The PING frame allows a client to safely test whether a connection is
still active without sending a request.