[API Stitching] Provide a hook to prepare responses based on remote server reponses

[sysadmins-gx]

When using API stitching, remote servers may incorporate specific headers that may be of interest/required to be propagated out for the client

In the same/similar way that API stitching allows to merge schema definitions so the response data is returned, our use case requires to have a mechanism to merge response headers.

Generalizing, I think it would be ideal to have a mechanism to prepare the final response from all remote server responses would be ideal. A couple of ideas:

• Advanced API to do responses merging.
• A way to pass responses information out so that a hook at the server level (express, koa, etc.) has a chance to do something (final response preparation) at the very end.

[yaacovCR]

See #969. Not sure if this can be streamlined.

[yaacovCR]

See #1380. Closing this for now as progress can be tracked there.