Stand-alone 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?

add-ons

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

It'd be great if I could create add-ons (eg. a PostgreSQL database) independent of an app instead of requiring add-ons always exist in the context of an app. Reasons include:

* Manage permissions independent of app permissiosn
* Get to have database for use without code/app (eg. for Heroku Connect)
* Complicated permissions when add-on is attached to apps owned by different accounts
* An app-owned add-on cannot be transferred to be owned by a different app in case the old app is going to be retired, or similar

Internally at Heroku this concept is called "Team Owned Add-ons" (as opposed to "App-owned"). It's a big change because the Heroku object model is currently so app-focused (eg. permissions, billing and so on all derives from app ownership)