delegateToSchema should return the full result with the extensions field for subscriptions

[hotgazpacho]

When returning a response to a subscription document, only the transformed data from the remote schema is returned.

https://github.com/apollographql/graphql-tools/blob/dacfd83566b6d8a9bee61ee1b5cda373b2b531a7/src/stitching/delegateToSchema.ts#L132-L134

This is a problem when the remote schema populates the websocket connection and channel subscription information in the extensions field of the response. I ran into this when attempting to merge a remote AWS AppSync schema into my Apollo Server lambda implementation. The way AWS AppSync implements subscriptions is via their IoT MQTT services. That is, when a request for a subscription comes in, as part of the response, it includes all the information _(such as the wss url, credentials, and channel metadata) in the extensions field in the response. AWS then provides a [SubscriptionHandshakeLink]( https://github.com/awslabs/aws-mobile-appsync-sdk-js/blob/master/packages/aws-appsync/src/link/subscription-handshake-link.ts) in their AppSync SDK that uses this information to extablish the websocket connection.

Without the extensions field being passed back up in the response, the client never receives the connection information.

[ghost]

This issue affects not only subscriptions, but also things like errors

dotansimha/graphql-binding#173

extensions field is missing when executing GraphQL operations like in https://github.com/graphql-binding/graphql-binding package which also uses delegateToSchema. As a result some sensitive information like Apollo error codes and other parameters stored in extensions are not retrieved when the operation is executed.

[yaacovCR]

See implementation in #969 using a link.