Flexible limits for number of domains per app

### 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

Tell us about what you're trying to solve. What challenges are you facing?
Customer are building customizable multi tenant applications on Heroku and would like to make it customizable including providing skins and custom domains. However they are restricted by the 1000 domains per app that we currently support. In order to meet their growing customer base they would like the number of domains to an app to be flexible so they can register a domain per customer.