Make case title description explicitly configurable from the source ZGW fields

[swrichards]

Taiga 2950

Now that there's three options, rather than two, to configure how a case title is displayed, the use_zaak_omschrijving_as_title flag has been deprecated in favor of an explicit enum:

As @alextreme pointed out earlier, this should really be a flag configurable on each individual API group rather than globally, but TLDR, that's a much larger refactor than it looks like, so we'll have to do that another day.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.39216% with 10 lines in your changes missing coverage. Please review.

Project coverage is 94.21%. Comparing base (7a79e99) to head (598d211).

Files with missing lines	Patch %	Lines
...kconfig_use_zaak_omschrijving_as_title_and_more.py	70.83%	7 Missing ⚠️
src/open_inwoner/openzaak/api_models.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1573      +/-   ##
===========================================
- Coverage    94.23%   94.21%   -0.02%     
===========================================
  Files         1073     1074       +1     
  Lines        39578    39621      +43     
===========================================
+ Hits         37297    37330      +33     
- Misses        2281     2291      +10     

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[pi-sigma]

I think the label for the first option in the new Enum is misleading and should be changed, unless I'm misunderstanding the current title/description situation.