Add Management Command for Async Ingredient Synchronization

[crypto-a]

Proposed Changes (Issue #1862 )

• Added a new management command python3 manage.py sync-ingredients-async to trigger the asynchronous ingredient synchronization task (sync_all_ingredients_task) via Celery.
• Updated the sync_all_ingredients_task decorator from @app.task to @shared_task for accessing the task from the manage.py commands.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR
  (e.g. database migration)?

• No, no additional commands or migrations are required.