Support the subscription operation type

[pkqk]

There are no current plans for implementing subscriptions.

This is a placeholder issue for discussing implementing subscriptions.

The main issue to consider is how long held connections (eg. via websockets) would affect the stateless design of the main bramble server process.

[gmac]

FWIW, this package claims to be an Apollo port with the inclusion of subscriptions, so might be worth a look for those concerned with subscription support https://github.com/jensneuse/graphql-go-tools.