[#2821] Implement provisional configuration for OpenKlant2 service

[pi-sigma]

The PR adds a provisional configuration for the OpenKlant2 service + tests so we can start integrating OpenKlant2 into our code. To be replaced with a proper configuration model.

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/task/2821

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.59%. Comparing base (0862012) to head (d4965e0).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/open_inwoner/openklant/models.py	85.71%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1452   +/-   ##
========================================
  Coverage    94.59%   94.59%           
========================================
  Files         1062     1063    +1     
  Lines        39651    39715   +64     
========================================
+ Hits         37507    37569   +62     
- Misses        2144     2146    +2     

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[swrichards]

Looks good, TLDR: let's try to not touch the openklant2 module at all if we can help it (except for the naming improvements you made in the client constructor).

[alextreme]

As long as we use the Services for the actual configuration later on, this is fine for now

[swrichards]

As long as we use the Services for the actual configuration later on, this is fine for now

That's indeed the idea (#1472 is a next step that in direction).