[FR] Built-in support pg_cron

I just want to perform some data cleanup or downgrade (e.g, downgrading user membership) periodically, but unfortunately, pg_cron is not part of the postgres standard extensions, so I can only do it by using a third-party docker image(or build one) for now.

There's no need to request a webhook outside of postgres to get something done, it's not only a waste of traffic, but also less robust

The issue names no repository files, tests, or entry points. Start by investigating how PostgreSQL extensions and scheduled jobs are currently handled, then clarify the intended pg_cron integration, configuration, and supported behavior with maintainers before defining what completion means.

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