Make Postgres Standard and Premium plans optionally not open to connections from public internet

### 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?

postgresql

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

Currently, all non-Private Space database plans can be connected to from the public internet (secured with encryption and requiring authentication). Only when moving to Private Spaces do we support the option of databases that are not open to the internet.

As an option, we should support creating databases used with Common Runtime apps that are not available on the public internet.