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]

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 ⚠️

Additional details and impacted files

@@            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              

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[mfolusiak]

tested with the default config from aptly(1)