Add external pull requests to Project Migration

What does this MR do and why?

Mentions #339409 (closed)

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

This MR adds External Pull 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.

It's also similar to our other BulkImports pipelines, like merge requests, issues, labels

Screenshots or screen recordings

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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 external pull request (I am not entirely sure how to do that)
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 external pull requests

MR acceptance checklist

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