[#2069] Upgrade KvK Zoeken API to v2

[pi-sigma]

Taiga: #2069

• Add validation to API config to ensure that the API root does not contain the version number and/or endpoint
• Add tests to ensure that API urls are constructed propery from the root
• Update existing tests and fixtures to use v2-compliant return values
• Refactor mocks to align better with OIP style
• Add data migration (strip version number from existing API roots)

[stevenbal]

If we're going to restrict the API root to these values, I think it's better to define choices for the field and let people select one of these four.

I'm trying to think of a case where we'd want to use something else but can't really come up with one, is it necessary though to restrict the API root to these four values? It would be a hassle if we have to do a new release just to allow a different value here 🤔

[pi-sigma]

I made the validation more general. I couldn't think of any reason for supporting other roots, but better safe than sorry.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f62132f) 94.83% compared to head (2ca5293) 94.90%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1016      +/-   ##
===========================================
+ Coverage    94.83%   94.90%   +0.06%     
===========================================
  Files          880      885       +5     
  Lines        30704    30766      +62     
===========================================
+ Hits         29119    29197      +78     
+ Misses        1585     1569      -16     

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[stevenbal]

Minor remark regarding mocks.simple. Looks good otherwise 👍

It might be useful for us at some point in the future to look into adding VCR.py (https://vcrpy.readthedocs.io/en/latest/), to deal with mocks and to make them more realistic instead of having to maintain them manually. Open Forms uses this as well

[stevenbal]

This is probably from another PR? Doesn't actually change anything though, so it's fine

[pi-sigma]

Double-checked: I branched off upgrade/django-admin-index instead of develop when starting on this. The PR has been merged though.