[#2700] Support multiple ZGW backends for zaak notifications

[pi-sigma]

• Support multiple ZGW backends for status notifications
• Support multiple ZGW backends for document notifications

Support for retrieving zaak notifications from different notification APIs is out of scope for this ticket

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/task/2700

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.23%. Comparing base (24881d7) to head (a2d28f5).
Report is 9 commits behind head on develop.

Files	Patch %	Lines
...n_inwoner/openzaak/tests/test_notification_data.py	90.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1370      +/-   ##
===========================================
+ Coverage    95.21%   95.23%   +0.02%     
===========================================
  Files         1004     1005       +1     
  Lines        37189    37249      +60     
===========================================
+ Hits         35408    35473      +65     
+ Misses        1781     1776       -5     

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