Issue #3205471 by cmlara, Berdir, deviantintegral: Stricter validation of TopicArn subscriptions?

Designed to allow backwards compatibility and hardening.

Creates a new configEntity that will retain the list of known subscriptions. Removes automatic approval requiring confirmation from an admin. Adds an option that new subscription requests be ignored (to avoid the filling up the configEntity list) Option to validate that the subscription is known before dispatching the message to subscribers. -- When validation is disabled a new subscriber will save attempt to save a new configEntity with each message received (the goal being that eventually an admin can feel confident to switch to requiring that it is a validated subscription)