Add support for setting either a custom domain or some kind of prefix (for PR Previews)

Using this github action to deploy on a PR I can almost get PR Previews setup (like heroku, netlify, render, etc).

The only roadblock that I'm running into is I need to know the URL ahead of time-- I need a predictable URL.

If we could set the URL through the apprunner step, I have access to the PR number inside the github action so it would be easy to formulate a url to pass in pretty easily.

It seems like maybe I can create my own action/step in the mean time using the API?

https://docs.aws.amazon.com/apprunner/latest/api/API_AssociateCustomDomain.html

Otherwise, only other missing thing to match heroku etc would be shutting down a service when a PR get's closed

Start by reviewing the App Runner deployment step and the AWS AssociateCustomDomain API linked in the issue. Determine how a predictable custom domain or prefix could be configured from the pull request number, and whether closing a pull request should also shut down its service; done means the chosen behavior is implemented and documented with validation for pull request previews.

Written by the indexing model from the issue text.