Extend ProcessWorkerActivity stats to get time average

Refs https://gitlab.com/teklia/requests/-/issues/787

We want to know the average time of processed worker activities for a worker version.

We'll extend ProcessWorkerActivity to add a float field average_processed_time that will calc the average time spent by worker activity (so avg(updated - created))

Bonus: I would also like to know if we can easily extend this to get:

• min & max times
• median time (percentile_cont(0.5))