Change model_config to allow extra fields

[GrandMoff100]

Closes #225

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (f18eedd) to head (910c18e).
⚠️ Report is 2 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #228   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files          25       25           
  Lines        1648     1648           
=======================================
  Hits         1606     1606           
  Misses         42       42           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.