[#2975] Consolidate feature flags for OpenZaak config

[pi-sigma]

• Move option to fetch eHerkenning zaken with RSIN from global OpenZaak config to ZGW API group config
• Add enum to ZGW API group config for selecting backend for klantgegevens

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/issue/2975

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.55224% with 7 lines in your changes missing coverage. Please review.

Project coverage is 94.19%. Comparing base (a5e4938) to head (b251599).

Files with missing lines	Patch %	Lines
src/open_inwoner/openklant/models.py	54.54%	5 Missing ⚠️
...pigroupconfig_fetch_eherkenning_zaken_with_rsin.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1575      +/-   ##
===========================================
- Coverage    94.22%   94.19%   -0.03%     
===========================================
  Files         1075     1079       +4     
  Lines        39657    39695      +38     
===========================================
+ Hits         37366    37391      +25     
- Misses        2291     2304      +13     

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