Move external_pull_requests to CI DB

What does this MR do and why?

This feature is tightly linked to CI as it's only useful for CI pipelines running against external pull requests so it makes sense to move it to the ci database.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348271 (closed)