Support Pong responses in websocket server

[BerkliumBirb]

graphql-ws Protocol describes Pong messages as response to Ping messages. Current server implementation doesn't support such responses.

[github-actions]

Looks like this issue has been open for 6 months with no activity. Is it still relevant? If not, please remember to close it.

[fordN]

Websocket connections to subgraphs are deprecated.