Simplistic Auth for Dev/Test/Staging apps

### 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?

Runtime, Routing

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

Currently there's no in-platform way to have Heroku add some sort of simplistic auth for apps. This might be useful to provide a measure of protection for non-production apps. Currently Heroku customers have to add this with language-specific libraries.

It'd be nice if Heroku could handle this concern in the platform. We could for example offer basic-auth+TLS where customer defines user-name / password and the check is implemented in the Heroku routing layer. Alternatively (or additionally) we could also offer stronger authentication schemes.