celery: allow using links instead of child spans for task execution (#4537)

* celery: allow using links instead of child spans for task execution

* Update instrumentation/opentelemetry-instrumentation-celery/src/opentelemetry/instrumentation/celery/__init__.py

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Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>