Implement load secrets from files.

[VovaStelmashchuk]

Resolve issue #289

I added a feature flag expandEnvFiles with a default value of false to ensure the package remains compatible with the previous version, in case the user already has an envvar with a suffix _FILE.

I assume the flag can be removed in a future major version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.40%. Comparing base (42d9697) to head (e3359ed).
⚠️ Report is 186 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   76.92%   78.40%   +1.48%     
==========================================
  Files           7        5       -2     
  Lines         806      352     -454     
  Branches       80      149      +69     
==========================================
- Hits          620      276     -344     
+ Misses        184       63     -121     
- Partials        2       13      +11     

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