Make relation export batch size configurable as an application setting

What does this MR do and why?

Make relation export batch size configurable as an application setting

This MR makes the batch size used by batched relation export configurable via application setting. This application setting can be updated via the REST API, and applies consistently across all batches for an export.

We cache the batch size for the duration of an export so that the cache size is consistent for a given export regardless of job restarts or settings changes.

Changelog: added

References

• #500862 (closed)

How to set up and validate locally

View the setting in the API

curl --header "PRIVATE-TOKEN: $GITLAB_DEV_TOKEN" \
  "http://gdk.test:3000/api/v4/application/settings" \
  | jq

Update the setting

curl --request PUT --header "PRIVATE-TOKEN: $GITLAB_DEV_TOKEN" \
  "http://gdk.test:3000/api/v4/application/settings?relation_export_batch_size=123"

Verify the import

1. Use Direct Transfer to import a project with more relations than your current export batch size.
2. Ensure all relations are properly imported.

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