Make logging level configurable

[tpoliaw]

Read initial log level from config file if present. Once the server is
running the level can be changed via a POST request to /loglevel/{level}
where level is one of info, debug or trace.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.88%. Comparing base (b6e2749) to head (2bd91fb).

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	18 Missing ⚠️
src/config.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
- Coverage   49.27%   47.88%   -1.39%     
==========================================
  Files          13       13              
  Lines         621      639      +18     
==========================================
  Hits          306      306              
- Misses        315      333      +18     

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