Support for external follower databases to Heroku Postgres

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku Postgres

### Tell us about what you're trying to solve. What challenges are you facing?

We're generally happy using Heroku Postgres for our database, and we have logical backups being captured, but we would like to be able to specify an external postgres system to act as a follower to our main database.

Currently, we can set another Heroku Postgres DB as a follower, but that keeps everything on Heroku, which is not ideal in the case of an outage (extended or otherwise).

Right now our only alternative to support redundancy is to entirely not use Heroku Postgres, where we can specify an external main database server to our app and then build followers from there, but we would prefer staying on Heroku Postgres as our main DB with the external follower as a backup.

Thanks to the team for all your work on such a great platform, been a customer since the early days!