Make the deletion of events from postgress configurable.

### Discussed in https://github.com/svix/svix-webhooks/discussions/1108

<div type='discussions-op-text'>

<sup>Originally posted by **cheleswer** October 19, 2023</sup>
Even after deleting the subscription, I see that events are still persisting in the Postgres DB.
Is there any certain time duration after which all these events will be deleted?  Is it configurable?</div>