[DOP-19794] Do not block event loop when using Celery

[IlyasDevelopment]

Change Summary

The call to the synchronous celery.send_task() method in the asynchronous endpoint is wrapped in asyncio.to_thread to avoid event loop blocking, in particular when RabbitMQ is unavailable.

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.03%. Comparing base (d465bdf) to head (ea996fd).
Report is 80 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #116   +/-   ##
========================================
  Coverage    93.03%   93.03%           
========================================
  Files          129      129           
  Lines         3088     3089    +1     
  Branches       241      241           
========================================
+ Hits          2873     2874    +1     
  Misses         157      157           
  Partials        58       58           

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