Allow users to skip exporting projects when exporting a group with offline transfer export
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=583383) </details> <!--IssueSummary end--> [Similar to Direct Transfer](https://docs.gitlab.com/api/bulk_imports/#start-a-new-group-or-project-migration), users should be able to skip exporting projects when exporting a group using offline transfer export. ## Proposed implementation - Add an `export_projects` option to the `entities` param in `POST /offline_exports` that defaults to true - When `export_projects` is false, all projects nested under the group entity should not be exported.
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