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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 74.76%. Comparing base (e564b7c) to head (2b61c40).

Files	Patch %	Lines
api/s3.go	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1247 +/- ## ========================================== - Coverage 74.78% 74.76% -0.03% ========================================== Files 143 144 +1 Lines 16200 16209 +9 ========================================== + Hits 12115 12118 +3 - Misses 3148 3154 +6 Partials 937 937 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.