Integrate with User Migration

[Pytal]

TODO drop all code for testing purposes and integrate with user_migration app API

[codecov]

Codecov Report

Merging #3917 (d741d1d) into main (dab0515) will decrease coverage by 0.36%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #3917      +/-   ##
============================================
- Coverage     29.61%   29.25%   -0.37%     
- Complexity      322      348      +26     
============================================
  Files           220      223       +3     
  Lines          7529     7622      +93     
  Branches        993      993              
============================================
  Hits           2230     2230              
- Misses         5299     5392      +93     

Flag	Coverage Δ
javascript	20.97% <ø> (ø)
php	63.16% <ø> (-4.19%)	⬇️

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Impacted Files	Coverage Δ
nextcloud/apps/calendar/lib/Command/Import.php	0.00% <0.00%> (ø)
nextcloud/apps/calendar/lib/Command/Export.php	0.00% <0.00%> (ø)
...ps/calendar/lib/UserMigration/CalendarMigrator.php	0.00% <0.00%> (ø)

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[tcitworld]

This should be in the dav app instead.

It will be pretty much the same for the Tasks and Contacts app, so the common parts should be together.

[Pytal]

Superseded by nextcloud/server#30963