Move creation of external cross-references into background worker

What does this MR do and why?

Calling external APIs can cause delays and timeouts, so to avoid this when creating cross references for external issues we move this processing into a dedicated background worker.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/343173

How to set up and validate locally

1. Set up an external issue tracker integration in a project (e.g. Jira).
2. Create an issue, commit, MR, or comment mentioning a Jira issue (e.g. DEMO-123, in title or description).
3. Observe the comment and link appearing in Jira on the mentioned issue.
   • Note that for comments, we link to the noteable (issue/MR/etc.) rather than the comment itself.
4. Do the same for other mentionables and observe no comment appearing: snippets, epics, designs, alerts, reviews.
   • This was also the case before this MR.

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 #343173