Allow users to skip exporting projects when exporting a group with offline transfer export
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[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.
issue