Calculate process time to completion

Refs https://gitlab.com/teklia/requests/-/issues/865

The endpoints ProcessWorkersActivity & CorpusWorkersActivity must be extended with an completion_estimate_time calculated as : ( remaining_worker_activity * median_processed_time )

• median_processed_time is self-explanatory and already calculated on this endpoint
• remaining_worker_activity is the number of remaining worker activity to process (non final state)