Import Command: Expose Tag Concurrency Option for S3

Context

The tag concurrency option allows faster import cycles after a pre-import. It could provide a significant reduction in import times for some customers on GCS.

Problem to solve

This was tested extensively on GitLab.com with GCS, but this option may cause greater instability with other drivers. AWS in particular has proven to resist performing higher levels of concurrency in offline garbage collection.

Proposal

Expose the tag concurrency option for S3-backed instances to help improve the performance of the offline migration after testing the stability of this option against a relatively large registry.