Improve promotional copy

[matthiasL-scality]

Purpose

Improve copy s3 to s3 (promote command)
This pull request updates the object copying logic in lua/copy_build.lua to improve efficiency and performance by introducing batch processing. Instead of copying objects one at a time, the code now processes them in batches using ngx.location.capture_multi, which allows multiple requests to be sent concurrently.

Performance improvements to object copying:

• Refactored the object copying loop to collect all object names up front and process them in batches of 16 using ngx.location.capture_multi for concurrent HTTP requests, instead of copying objects one at a time.
• Updated progress output and flushing logic to reflect batch processing and ensure correct progress reporting.

[bertranddemiddelaer]

Nice to have: I did not see a test regarding the promotion of a build. This would be really nice to have in order to fully validate those optimizations.

[matthiasL-scality]

Nice to have: I did not see a test regarding the promotion of a build. This would be really nice to have in order to fully validate those optimizations.

https://github.com/scality/artifacts/pull/213/changes#diff-019e68f115c410de17a78789e0eaaf5260ae05195c2e21714eb07983f8998ab4R74-R88 should do the job

[bertranddemiddelaer]

LGTM