feat(Subscriptions) add subscriptions

[rmosolgo]

Let's support GraphQL Subscriptions in Ruby!

Some library changes are required:

• Special behavior for executing subscriptions:
  • On the first execution, no fields are executed. Instead, each root field + arguments pair is registered with the subscription backend.
  • On later executions, only root field + argument pairs which match the triggered event are executed. The root value for that execution is an application-provided object. (eg, if commentAdded is triggered, the root object is the Comment provided by the application.)
  • Add Query::Result to simplify re-evaluation feat(Query::Result) add a first-class result object #898

My implementation has made an assumption about how root fields map to application events:

event_key = "#{field.name}(#{JSON.dump(args.to_h)})"
# commentAdded({"postId":1})

Is this sufficient for capturing the "unit of subscription"? Or does it miss some cases?

See the guides in this PR for documentation.

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Working demo: https://gist.github.com/rmosolgo/ba31acf93f07f8007d99ba365a662d8f

[rmosolgo]

Feature-wise I think this is ready, but as I write the docs, I think the API can be better. Something more ... object-oriented 🙀

[rmosolgo]

Working on an ActionCable client rmosolgo/graphql-ruby-client#2

[blevine]

@rmosolgo Not quite sure what the status of subscription support is at this point. Would I be able to pull the 1.7.0 branch to start implementing subscriptions now?

[rmosolgo]

@blevine not yet, i'm hoping to merge this into 1.7 today

[rmosolgo]

Here goes nothin! Feel free to take the 1.7 branch for a spin.