add private key auth method support for SFTP

[CaptainOfHacks]

No description provided.

[swarmia]

✅  Linked to Task TED-1247 · update the SFPT adapter to work with certificate-based authentication
➡️  Part of Epic TED-1041 · Improvements (P3)

[codecov-commenter]

Codecov Report

Base: 96.27% // Head: 96.29% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (d64ac9d) compared to base (639d940).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
+ Coverage   96.27%   96.29%   +0.01%     
==========================================
  Files         106      106              
  Lines        5562     5582      +20     
==========================================
+ Hits         5355     5375      +20     
  Misses        207      207              

Impacted Files	Coverage Δ
...notice_publisher/adapters/sftp_notice_publisher.py	100.00% <100.00%> (ø)

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