Move spec/frontend_integration/diffs/diffs_interoperability_api.js to its own package

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Description

Currently we are hardcoding the way third-parties like Sourcegraph should interact with our MR diffs page.

!58746 (diffs)

Proposal

Let's move this into it's own package so that both Sourcegraph and GitLab can consume the package and make sure we're using the same thing.

Maybe we can call this package @gitlab/interop 🤷