Add optional_callback `handle_ping`

[bnhansn]

The graphql-ws spec mentions a Ping message may be useful for detecting failed connections or other types of network probing.

To allow an application to hook into this feature, this PR adds:

• An optional handle_ping callback in the socket implementation.
• Updates the Pong response to include the same payload that was sent in the Ping.

[aloukissas]

Hey @sax - what can we do to merge this? This seems useful.