Add Push Rule to Project Migration

What does this MR do and why?

Related to #339413 (closed)

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

This MR adds Push Rules migration when using Group Migration that includes projects (https://docs.gitlab.com/ee/user/group/import/).

Screenshots or screen recordings

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. Update project push rules in the 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 push rules got imported.

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.