feat(worker): Add backup remote for exported data

[nellh]

Adds a new git-annex remote called s3-BACKUP to provide a second copy of all annexed objects for redundancy with s3-PUBLIC.

[codecov]

Codecov Report

❌ Patch coverage is 67.74194% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.96%. Comparing base (6dc4585) to head (7bb2074).
⚠️ Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
services/datalad/datalad_service/tasks/publish.py	40.00%	6 Missing ⚠️
services/datalad/datalad_service/common/s3.py	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3604      +/-   ##
==========================================
+ Coverage   42.93%   42.96%   +0.02%     
==========================================
  Files         633      633              
  Lines       33142    33169      +27     
  Branches     1479     1479              
==========================================
+ Hits        14231    14250      +19     
- Misses      18772    18780       +8     
  Partials      139      139              

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