Add GraphQL Interface (Apollo) to Diffs App
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[From this comment](https://gitlab.com/gitlab-org/gitlab/-/issues/322798#note_524582021).
Based on the [intended backend implementation](https://gitlab.com/gitlab-org/gitlab/-/issues/322798#note_524066907), the ~frontend will need to add a GraphQL interface to the Diffs app, which doesn't yet exist.
This is a complicated addition, especially considering:
- It will add significant additional code to the application bundle
- It will add another "storage" location where data is cached
- It's an entirely new "way" of connecting to the backend for the Diffs app
issue