feat: new datasources: host_key_files

[shlao]

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Complete Description of Additions/Changes:

Add datasource to check why sshd couldn’t load host key.

[xiangce]

It's not a good idea to read the /var/log/messages directly in datasource as it would be huge.
If you have to do that, please use the existing spec, as it's filtered.

Finally, if the file on customer side is huge even after filtering, this datasource will be time/memory consuming.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.51%. Comparing base (77360f5) to head (156950e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4415   +/-   ##
=======================================
  Coverage   77.51%   77.51%           
=======================================
  Files         744      745    +1     
  Lines       41585    41610   +25     
  Branches     8832     8837    +5     
=======================================
+ Hits        32233    32256   +23     
- Misses       8318     8320    +2     
  Partials     1034     1034           

Flag	Coverage Δ
unittests	77.50% <100.00%> (+0.01%)	⬆️

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[xiangce]

@shlao as mentioned, we need to reconsider if it's worth adding this spec to core collection, do you have a Jira card to get this spec being approved first?