sftp: support for password authentication

[jkowalski]

Fixes #1373

[codecov]

Codecov Report

Merging #1425 (550c093) into master (3c4cf33) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1425      +/-   ##
==========================================
- Coverage   68.38%   68.37%   -0.02%     
==========================================
  Files         366      366              
  Lines       28884    28895      +11     
==========================================
+ Hits        19752    19756       +4     
- Misses       7485     7490       +5     
- Partials     1647     1649       +2     

Impacted Files	Coverage Δ
repo/blob/sftp/sftp_options.go	50.00% <ø> (ø)
cli/storage_sftp.go	83.78% <100.00%> (+1.69%)	⬆️
repo/blob/sftp/sftp_storage.go	56.90% <100.00%> (+0.49%)	⬆️
cli/command_content_verify.go	73.68% <0.00%> (-5.27%)	⬇️
snapshot/snapshotfs/estimate.go	96.66% <0.00%> (-3.34%)	⬇️
internal/cache/persistent_lru_cache.go	84.17% <0.00%> (+1.89%)	⬆️

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