GraphQL Stitching is losing "extensions" information from sub-schema response

### Issue workflow progress

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- [ ] 1. The issue provides a reproduction available on Github, Stackblitz or CodeSandbox
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> Please make sure the GraphQL Tools package versions under `package.json` matches yours.
- [ ] 2. A failing test has been provided
- [ ] 3. A local solution has been provided
- [ ] 4. A pull request is pending review

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**Describe the bug**

Stitching a remote schema loses any "extensions" information in the query response of a sub-schema

A GraphQL query response may have a top-level "extensions" field next to the "errors" and "data" fields:

https://spec.graphql.org/June2018/#sec-Response-Format

This "extensions" section can be used to provide auxilliary information such as profiling information or hints back to callers for performance optimization, or indeed anything else.

This information is lost when stitching in the remote graphql schema into a stitched global schema.

I would expect to be able to provide a merge function for the optional 'extensions' part of the response payload, with a default implementation which would naively merge the map contents.

No source files, tests, reproduction, or entry points are identified in the issue. Start by creating the requested GraphQL schema-stitching reproduction and inspect how a sub-schema response is handled; done means the top-level extensions information is preserved or merged in the stitched response, with a failing test demonstrating the behavior.

Written by the indexing model from the issue text.