Add GraphQL Interface (Apollo) to Diffs App
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=324112) </details> <!--IssueSummary end--> [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
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