feat(settings): allow custom encoding for dotenv files

[PrettyWood]

Change Summary

Allow to customize encoding for dotenv files

Related issue number

closes #1615

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1620 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1620   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3796      3798    +2     
  Branches       752       752           
=========================================
+ Hits          3796      3798    +2     

Impacted Files	Coverage Δ
pydantic/env_settings.py	100.00% <100.00%> (ø)

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[samuelcolvin]

great, thank you.