Support overages for Heroku Add-ons

### 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 Add-ons

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

Event based add-ons need automatic overages so that customers can continue to use the add-on service after they've reached their add-on plan's event limit.

For example, if a customer purchases an add-on plan for error logging, and hits their limit on the 20th of the month, they will not benefit from the add-on finding errors for them until the next month starts.