Improve offline transfer export logging
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## Summary
Currently, when relation exports process, logs include information like:
```json
{
"severity": "INFO",
"time": "2026-02-13T22:30:03.376Z",
"correlation_id": "01KHCJ05AE0N2AJYP41BJAXNW2",
"meta.caller_id": "BulkImports::RelationExportWorker",
"message": "Exporting relation: labels",
"importer": "Import/Export",
"group_id": 38,
"group_name": "Flightjs",
"group_path": "flightjs",
"relation": "labels",
"number_of_records": 10
}
```
However, there's no information differentiating logs generated by direct transfer exports and offline transfer exports. For better debugging as offline transfer is implemented, add the following to offline transfer logs:
- `offline_export_id` of the `Import::Offline::Export` the relation export belongs to
- `import_type`
## Related
- Related to [!218532](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/218532) - Create offline transfer export worker
- Related to [#576092](https://gitlab.com/gitlab-org/gitlab/-/issues/576092)
issue