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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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%)	:arrow_up:

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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?