s3 api: get publish root list

[neolynx]

Description of the Change

allow to query the available s3 buckets, so the information can be used by subsequent api calls.

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