Pipeline Links: Add hover tip callout to backend

What does this MR do?

This is the backend MR for adding a one-time tip about hovering to the pipeline graph job dependencies (needs/layers) view as outlined in #326287 (closed). Because it is GraphQL, it needs to ship before the frontend to avoid mixed deploys.

Screenshots (strongly suggested)

No visible change.

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because is it partial and umimportant.
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Related to #326287 (closed)