Group export/import status API endpoint
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### Problem to solve
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[Group export/import](https://docs.gitlab.com/ee/api/group_import_export.html) API lacks a status endpoint similar to that found in Project [export](https://docs.gitlab.com/ee/api/project_import_export.html#export-status)/[import](https://docs.gitlab.com/ee/api/project_import_export.html#import-status).
### Intended users
This feature is requested by our premium customer via this [ZD ticket](https://gitlab.zendesk.com/agent/tickets/151444) (internal)
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