[configoptional] Add GetOrInsertDefault method

[mx-psi]

Description

Adds GetOrInsertDefault method to configoptional.Optional. Uses it in tests.

We originally did not do this because we only had use cases for this in tests. However #13856 shows that this is useful in programmatic use cases as well.

Link to tracking issue

Fixes #13856

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.37%. Comparing base (cb24802) to head (a9b14ca).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
config/configoptional/optional.go	75.00%	2 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (75.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13859   +/-   ##
=======================================
  Coverage   91.36%   91.37%           
=======================================
  Files         648      648           
  Lines       42626    42636   +10     
=======================================
+ Hits        38947    38960   +13     
+ Misses       2855     2852    -3     
  Partials      824      824           

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[atoulme]

thank you!