Add designs migration to GitLab Projects Migration

What does this MR do and why?

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

This MR adds design migration to GitLab Project Migration feature (https://docs.gitlab.com/ee/user/group/import/). The implementation is identical to our traditional file-based Project Import/Export's design repo saver & design repo restorer.

Mentions #339421 (closed)

Screenshots or screen recordings

withdesigns2

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 an issue and upload designs to it
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 designs. They should be in the issue.

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.