[#900] Configuration openklant

[SilviaAmAm]

Fixes #900

Depends on #921

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.95960% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.39%. Comparing base (380ffdd) to head (7bfa389).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/openarchiefbeheer/external_registers/plugin.py	89.13%	5 Missing ⚠️
...eer/external_registers/contrib/openklant/plugin.py	88.88%	3 Missing ⚠️
...c/openarchiefbeheer/external_registers/registry.py	91.30%	2 Missing ⚠️
...ers/contrib/openklant/setup_configuration/steps.py	94.73%	1 Missing ⚠️
...ngs/pages/health-check/HealthCheckSettingsPage.tsx	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #917      +/-   ##
==========================================
+ Coverage   90.28%   90.39%   +0.10%     
==========================================
  Files         214      227      +13     
  Lines        6885     7119     +234     
  Branches      696      704       +8     
==========================================
+ Hits         6216     6435     +219     
- Misses        669      684      +15     

Flag	Coverage Δ
backend	91.62% <96.07%> (+0.16%)	⬆️
jest	34.11% <11.76%> (-0.19%)	⬇️
storybook	83.48% <94.11%> (-0.05%)	⬇️

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[SilviaAmAm]

While working on #901 I am considering changing this to a non-solo model.

Then each plugin would be related to a specific instance of a PluginConfig model with a specific identifier (a bit like we have for the mozilla-django-oidc-db package).

This has the advantage that if we want to go from the Service to the plugin, we can do :

config = Service.pluginconfig_set.first()
plugin = register[config._plugin_identifier]

instead of having to figure out which many-to-many accessors there are on the Service that correspond to plugin configuration models and then which of these querysets is non empty 🤔