Add MR reviewers to Project Import/Export

What does this MR do and why?

This MR adds MR reviewers information to Project Import/Export, so that when project is migrated, the merge request reviewers are preserved.

Mentions #339520 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Create project with a merge request
2. Invite project members and assign them as reviewers
3. Update project members to have their public email displayed
4. Export project using Project Export functionality (under Settings -> Advanced)
5. Download exported tarball and import it back
6. Verify imported project has merge request imported & members assigned as reviewers to it

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.