feat: support environment variable substitution in config files

[mbezhanov]

Hi.

This PR implements the changes suggested in #2844.

The proposed format is ${VARIABLE} like in the envsubst Linux command, but I can tweak it however you like.

If any further changes are necessary, I'll be happy to add them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.55%. Comparing base (f997fb9) to head (2a618a2).
Report is 427 commits behind head on main.

❗ Current head 2a618a2 differs from pull request most recent head 2fab1f5

Please upload reports for the commit 2fab1f5 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3195      +/-   ##
==========================================
+ Coverage   70.78%   71.55%   +0.76%     
==========================================
  Files          91      109      +18     
  Lines        8729     8004     -725     
==========================================
- Hits         6179     5727     -452     
+ Misses       2165     1848     -317     
- Partials      385      429      +44     

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[markphelps]

looks great @mbezhanov !! thank you for the contribution! would you mind going through the step required to sign the DCO?

https://github.com/flipt-io/flipt/pull/3195/checks?check_run_id=26421002517

[markphelps]

@all-contributors please add @mbezhanov for code

[allcontributors]

@markphelps

I've put up a pull request to add @mbezhanov! 🎉

[mbezhanov]

looks great @mbezhanov !! thank you for the contribution! would you mind going through the step required to sign the DCO?

https://github.com/flipt-io/flipt/pull/3195/checks?check_run_id=26421002517

Done! 🙂