Feat: Add a `registered_on` column to `user_reg.db`

My understanding is the last_update column could potentially be overwritten by a repeat registration action. That should never happen, but just in case...the registered_on column would contain the date of initial registration for each user.

Having a definitive source of this date is useful for metrics, reporting and facilitation.

mentioned in merge request rabbitmq_agents!145 (closed)

Not going merging PR right now.

Because we are adding new column in the database, need to see how we can set default value for existing rows.

assigned to @louistw

added Sprint 24-7 label

closed

moved to rabbitmq_agents#152