Limit concurrent BulkImports::PipelineBatchWorker

What does this MR do and why?

Limit concurrent BulkImports::PipelineBatchWorker

This limits the number of concurrent in-progress BulkImports::PipelineBatchWorker workers that will run for a single Direct Transfer migration.

https://gitlab.com/gitlab-org/gitlab/-/issues/429863

How to set up and validate locally

1. Enable the flag (Feature.enable(:bulk_import_limit_concurrent_batches)).
2. Enable batched exports (Feature.enable(:bulk_imports_batched_import_export)).
3. Enable direct transfer under Import and export settings.
4. Enable local -> local transfers (ApplicationSetting.first.update(allow_local_requests_from_web_hooks_and_services: true)).
5. Import an existing group from your instance into another namespace.
6. Confirm that the transfer completes successfully.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #429863