Subscription API#75

Rather than having to rely on a combination of query and update queries, it would be great to be able to simply subscribe to a query and have the UI update automatically.

Allowing Subscriptions would be a unique selling point compared to other CMS solutions using GraphQL!

To create previews for editors this would be a great feature!

This feature would be huge! Feels like the only major edge Sanity has over GraphCMS (see their docs: https://www.sanity.io/docs/realtime-updates)

Wouldn’t this be the same as Webhooks?