Support subscriptions in the GraphQL API

First of all big shout-out for the new GraphQL API support in Cube, with this we now have a single layer for both transactional and analytics APIs. (REST in peace).

But it would be really cool if we had a support for real-time queries with streaming results over GraphQL + websocket transport.

Possible solution would be to use:
[express graphql subscription support](https://github.com/graphql/express-graphql#setup-with-subscription-support)
as this dependencies is already provided.