Add offline transfer application settings (admin level)
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## Summary
Add admin-level application settings to control offline transfer exports and imports, similar to other importer settings in `admin/application_settings/general`.
As part of the offline transfer feature for semi-automated migrations between offline instances (&8985), administrators need to be able to enable/disable offline transfer functionality at the instance level.
### Backend
- Create application settings to gate offline transfer exports
- Create application settings to gate offline transfer imports
- Settings should be stored in `admin/application_settings/general` alongside other importer settings
- Settings should be retrievable and updatable via API
### Frontend
- Add UI controls in admin settings panel to manage offline transfer settings
- Display toggles/controls for (these can be refined further with Technical Writing)
- "Allow exporting GitLab groups and projects by offline transfer"
- "Allow importing GitLab groups and projects by offline transfer"
- Add tasks to the documentation placeholder issue https://gitlab.com/gitlab-org/gitlab/-/issues/581225
## Notes
- These settings should follow the same pattern as other importer settings already in the application
- Settings should control both the API availability and UI visibility of offline transfer features
## Related Issues
- &8985 - Support semi-automated migrations between offline instances
## Acceptance Criteria
- [ ] Application settings for offline transfer exports are created and stored
- [ ] Application settings for offline transfer imports are created and stored
- [ ] Settings can be retrieved and updated via API
- [ ] Admin UI displays offline transfer settings in `admin/application_settings/general`
- [ ] Settings properly gate the offline transfer functionality
- [ ] Documentation is planned for before the API release and the subsequent UI release
issue