Add a property allowing to get task progress from celery in CeleryTask entity class.

This implies that the celery tasks must regularly update their status with the
following structure : {'progress': progress_ratio}, with progress_ratio being
a floating number between 0 and 1.

If it is not the case, the progress bar will pass from grey to blue in one go, when
the task state passes to 'done'.

closes #14983709