[GRAPH-1102] Customizable doc storage with default implementation

[bryanjos]

• Create a behaviour named Absinthe.Subscription.DocumentStorage to allow customization around how Absinthe stores subscription documents and field key pairs.
• Create an implementation named, Absinthe.Subscription.DefaultDocumentStorage implemented using the Registry and algorithms Absinthe used previously
• Update Absinthe.Subscription.Supervisor to check for a storage key in its opts. The default value is Absinthe.Subscription.DefaultDocumentStorage
• Update Absinthe.Subscription to use the configured storage module

[GeoffreyPS]

I left some feedback on things that confused me or seemed a little awkward.

I think exposing this as a behavior is a great idea, so I agree with the direction you're taking this.

[bryanjos]

@benwilson512 @maartenvanvliet Hey we're looking to add support for customizing how Absinthe stores subscription documents in our fork. But we also want to make sure it's going in a direction that it can be added into Absinthe proper. Do you all mind looking and giving any feedback?

[bryanjos]

I think I might have overthought this. Instead, people can just implement the behaviour, tell Absinthe what module to use and then they can start any processes they need before starting Absinthe.Subscription
#11