Multiple app processes exposed to the web

### 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?

Within a single app, we should support multiple process types that are exposed to the internet. Currently, a Heroku app can only have one process type (`web`) exposed to the web. Other process types are worker processes or one-offs.

Customers need multiple web-type processes because they have monolithic codebases that include multiple microservices or web services (eg. website and API in same codebase) and those need to be able to scale independently. We could make that easier to deploy to Heroku if we supported multiple web processes within a single app.

There are a lot of things that would have to get figured out:

* default hostnames
* certificate management and mapping
* buildpack conventions
* ... more