The next step for the GitLab Migration feature is to support
also Project migrations. Right now, only Group Migrations are
available as per: https://docs.gitlab.com/ee/user/group/import/

Therefore, 3 API endpoints are being added:

POST /projects/:id/export_relations will support triggering the async
export into gz files that contain ndjson compacted files for each
relation exported.

GET /projects/:id/export_relations/download will support downloading
the async exported files if they are already available.

GET /projects/:id/export_relations/status will support verifying
whether the relations have been successfully exported already or not.

Additional technical notes:

- Added a missing test to group bulk_import_exports association.
- Added project bulk_import_exports association.

Changelog: added