Add support for the GraphQL-WS Subprotocol

[artran]

I've extended the Websocket endpoint to support the newer GraphQL-WS sub-protocol in addition to the existing ApolloWS sub-protocol.

Apollo-WS: https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md
GrapQL-WS: https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md

My changes are covered by a single, small, tested commit and I'd be glad to create a pull-request if you think the change would be generally useful.

Details: artran@60a93b5

Cheers,
artran