✨ [#2157] Implement newsletter subscription form

[stevenbal]

https://taiga.maykinmedia.nl/project/open-inwoner/us/2157

Can be tested locally by:

• configuring in the admin with the with the information from the docs (no basic auth password needed): https://api.laposta.nl/doc/
• enabling Newsletters in the ProfileApphookConfig and navigate via Mijn profiel (or directly by navigating to http://127.0.0.1:8000/mijn-profiel/newsletters)

I introduced pydantic because:

• because Laposta is not a ZGW API, it felt a bit strange to use zgw_consumers ZGWModels and factory for this API
• zgw-consumers itself has an open issue to switch to pydantic (Switch to pydantic zgw-consumers#51)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 67.04545% with 58 lines in your changes are missing coverage. Please review.

Project coverage is 94.84%. Comparing base (98dbb93) to head (e9c9107).
Report is 6 commits behind head on develop.

Files	Patch %	Lines
src/open_inwoner/laposta/client.py	38.29%	29 Missing ⚠️
src/open_inwoner/laposta/forms.py	36.36%	21 Missing ⚠️
src/open_inwoner/accounts/views/profile.py	54.54%	5 Missing ⚠️
src/open_inwoner/laposta/models.py	86.36%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1082      +/-   ##
===========================================
- Coverage    95.00%   94.84%   -0.16%     
===========================================
  Files          894      903       +9     
  Lines        31404    31580     +176     
===========================================
+ Hits         29834    29952     +118     
- Misses        1570     1628      +58     

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[Bartvaderkin]

Pretty clean and simple, Pydantic seems like a good idea.

I got some notes on some peculiarities (and some small stuff).

[stevenbal]

@pi-sigma could you re-review this? :)