Use the newer recommended S3 ListObjectsV2 API

[giuli007]

This change is to address a situation we have encountered running in AWS EKS where each single (1000-paginated) request using the older ListObjects API via s3.ListObjectsPages takes seconds to receive a response, resulting in tens of minutes long publishing times when the bucket contains >100k packages (which is the cases for repos mirrored from ubuntu repos and republished in-house).
This doesn't look like normal throttling as no explicit throttled responses are seen when enabling debug in the https://www.aptly.info/doc/configuration/. The reasons why the old API is so slow only when running in EKS are still unknown.

Description of the Change

Uses the more modern recommended ListObjectsV2 to list objects on the S3 bucket when publishing to it.
https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListObjectsV2.html

This is faster, especially when running in EKS.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[neolynx]

this made it into master already..

thanks !