Expand extended interface selections for subservice compatibility

[gmac]

Resolves #1911. (@yaacovCR)

As discussed in #1911, extended gateway interfaces are incompatible with the subservice below them. Thus, interface extensions need to be expanded for their possible types.

This is implemented using the following steps:

1. Build memoized lists of interface field extensions, which are detected by comparing the gateway fields to the subservice implementation.
2. While visiting selections, also check if a selection falls within an extended interface. If so, mark it.
3. Filter all fields marked as interface extensions out of the selection, and add them back in as expanded type fragments.

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[gmac]

Everything is passing here aside from that canary message, which @ardatan has indicated is a known issue. Ready for another look.

[yaacovCR]

Thanks so much!!!

[gmac]

Mind pushing out the docsite if you have a minute?

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.0.19-alpha-91839410.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.0.19-alpha-91839410.0