Allow CI runs from public fork

### 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?

CI

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

If a contributor to my repository makes a pull request via a fork, I cannot run CI without opening my own pull request, in my repo, of their branch. This clutters github and makes code reviews complicated.

It seems a [similar feature was removed in 2017](https://devcenter.heroku.com/changelog-items/1208).

Github has an option to "Approve" workflow runs from untrusted public forks. I would appreciate if Heroku CI had such an option as well.

Some ideas:
1. Respect the github action approval flow. If the GH Action runs, so too should Heroku CI.
2. Allow a custom github action to trigger the Heroku workflow. This is the most work for us, but probably the easiest for you: we'd just craft an API call to heroku to trigger the workflow run via github actions.
3. Manually trigger it via the Heroku UI. This is the most annoying as it's not automated, but at least it would make it possible.