Heroku Postgres Add-on Slack notifications

### 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?

Postgres

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

Right now there is no way to get real-time Slack notifications about db health state (connections, data size, pooling, maintainance) except emails and `heroku pg:info`.

Ideally there should be either a built-in Slack notifiction module, or API available.