Fine-grain access to protect app/remove UI changes to settings

### 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 dynos

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

The same way Github has a feature to "protect a main branch" to avoid unexpected and uncontrolled operation on this important branch, it would be nice for Heroku to let admins "protect an app" to disable changes to the app settings via the UI.

For example, in a pipeline where you have a development version of the app, a staging version and a production version :
- in the development app, settings should be UI-changeable, as is the fact today
- in staging and production versions of the app, admins could change an app-permission to disable the possibility for admins to change settings via the UI.

These settings would be at least config vars and buildpacks.
The idea is to control changes to these settings and make sure that they are traced either via changes to a version-controlled file (such as app.json) or that they are triggered by a CI operation via the Heroku CLI (or API integration with the Heroku API).

Admins could be able to change these settings themselves, which would mean, indeed, that they could toggle this access permission off then on again to change something via the UI; the idea of this feature is to avoid errors.
For security-centered organizations, it could be possible to remove the possibility for some non-super admins to change these app access permissions.