Add merge requests to Project Migration

What does this MR do and why?

Mentions #339403 (closed)

⚠ this feature is behind bulk_import_projects feature flag that is currently off by default (and disabled on .com).

This MR adds Merge Requests migration when using Group Migration that includes projects (https://docs.gitlab.com/ee/user/group/import/). It's doing exactly the same thing as file-based Import/Export by using it's RelationFactory within NdjsonPipeline which is an Import/Export class.

mr-migration

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

1. Feature.enable(:bulk_import)
2. Feature.enable(:bulk_import_projects)
3. Create a top level group and project in it
4. Create merge requests in newly created project
5. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api & read_repository scope)
6. Select newly created group and click Import
7. Wait for Group import to complete and verify imported project's issues

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.