Support deletion events in the outbox routing SMT

Currently, there's no way to signal when outbox events can be deleted from the Kafka topics. This is because the fact that an event record in the outbox table got deleted doesn't mean that the events for the same aggregate are deleteable, which is why the outbox routing SMT discards tombstone events atm.

To support deletions, there should be new supported column "is deleted", which would be set to true by applications writing to the outbox table when producing an event that represents the deletion of an aggregate. If true, the outbox router will take this as the signal to produce a tombstone event.