Auth using environment variables

[characat0]

This PR modifies the default constructor to read environment variables MLFLOW_TRACKING_USERNAME and MLFLOW_TRACKING_PASSWORD for credentials and creates the Authorization header using basic auth.

[pebeto]

LGTM, but tests are required for almost every PR. Don't forget to add the new env variables here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.39%. Comparing base (7067af0) to head (0a92b04).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   89.18%   90.39%   +1.20%     
==========================================
  Files          19       21       +2     
  Lines         333      406      +73     
==========================================
+ Hits          297      367      +70     
- Misses         36       39       +3     

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