Refactor celery tasks + management commands

[pi-sigma]

Some of our celery tasks are wrappers around management commands - a legacy feature of the earlier implementation:

• https://github.com/maykinmedia/open-inwoner/blob/develop/src/open_inwoner/configurations/tasks.py
• https://github.com/maykinmedia/open-inwoner/blob/develop/src/open_inwoner/openzaak/tasks.py
• https://github.com/maykinmedia/open-inwoner/blob/develop/src/open_inwoner/search/tasks.py

The PR improves task logging by including the output of the management commands in the celery task results. We decided to keep the commands around for the time being because the libraries we currently use for manually triggering tasks are not maintained, so we may need to make use of the management commands after all.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.13%. Comparing base (8f22d47) to head (b2f7e8c).
Report is 15 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1347      +/-   ##
===========================================
+ Coverage    95.11%   95.13%   +0.01%     
===========================================
  Files          995      998       +3     
  Lines        36472    36601     +129     
===========================================
+ Hits         34692    34819     +127     
- Misses        1780     1782       +2     

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