[#9] Make retrieval of Open Forms choices lazy

[pi-sigma]

Issue (Open Inwoner): https://taiga.maykinmedia.nl/project/open-inwoner/issue/2236

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.04%. Comparing base (1c90c11) to head (a9351a6).
Report is 3 commits behind head on main.

❗ Current head a9351a6 differs from pull request most recent head 105d753

Please upload reports for the commit 105d753 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   95.03%   95.04%           
=======================================
  Files          20       20           
  Lines         504      505    +1     
=======================================
+ Hits          479      480    +1     
  Misses         25       25           

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[stevenbal]

Looks good and works as expected, could you also add a test that initializes the field twice and checks that the network call is only done once (and done again once the cache key is invalidated)? :)