Feat/Automatically find free slots for an event

[GVodyanov]

Close https://github.com/nextcloud/calendar/issues/5697

TODO:

• [x] Create service
• [x] Create separate function to limit repetition between service and freeBusyResourceEventSource.js
• [ ] Implement the automatic slot search (basic mechanism done)
• [ ] Create dedicated UI components in the popover
  • [ ] Possibly in the sidebar too
• [ ] Algorithm optimizations
• [ ] Unit tests

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 126 lines in your changes are missing coverage. Please review.

Project coverage is 23.73%. Comparing base (0176063) to head (d47632e). Report is 4 commits behind head on main.

Files	Patch %	Lines
src/services/freeBusySlotService.js	0.00%	101 Missing :warning:
src/components/Editor/FreeBusy/FreeBusy.vue	0.00%	20 Missing :warning:
...lendar/eventSources/freeBusyResourceEventSource.js	0.00%	5 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5799      +/-   ##
============================================
- Coverage     23.98%   23.73%   -0.26%     
  Complexity      423      423              
============================================
  Files           243      244       +1     
  Lines         11105    11223     +118     
  Branches       1839     1862      +23     
============================================
  Hits           2664     2664              
- Misses         8441     8559     +118     

Flag	Coverage Δ
javascript	14.76% <0.00%> (-0.20%)	:arrow_down:
php	61.97% <ø> (ø)

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