A subscription field's resolver pair.

subscribe returns the async iterable the field streams from and is attached to the schema field's subscribe slot; the optional resolve maps each emitted payload to the field value, exactly as graphql defines it.

Without this arm a subscription is unexpressible in the schema-first map: an entry has to be a function, and assigning { subscribe } to a field's resolve leaves subscribe unset, which makes graphql.subscribe() throw "Subscription field must return Async Iterable".