Improved Terraform support for Heroku

### 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?

API

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

A Terraform provider exists for Heroku, and we have [Dev Center documentation covering its use](https://devcenter.heroku.com/articles/using-terraform-with-heroku).

We often get feedback from customers that they want better Terraform support. I'm opening this issue to try and collect that feedback.

The maintainers of the Terraform provider tags issues for missing features due to missing APIs here: https://github.com/heroku/terraform-provider-heroku/issues?q=is%3Aopen+is%3Aissue+label%3A%22Heroku+API+Support%22

An additional HerokuX provider provides features that are not covered in the official Heroku API: https://github.com/davidji99/terraform-provider-herokux

Looking at the "resources" component in the terraform docs for HerokuX is instructive to understand gaps in Heroku's API: https://registry.terraform.io/providers/davidji99/herokux/latest/docs