Additional environments for Pipelines

### 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 Pipelines

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

We would like the ability to add more vertical (as shown in the Pipelines UI) environments to our Heroku Pipelines, so that we could have more environments to promote code through before reaching Production.

Currently with only 3 environments in Heroku Pipelines (Development, Staging, Production, Review Apps), we are limited in the number of work streams we are able to have going in our code while still following proper code promotion processes.

In the Pipelines doc FAQ, it's made clear that no other environments are currently supported: https://devcenter.heroku.com/articles/pipelines#can-i-have-pipelines-stages-other-than-development-staging-and-production