Modify branch name to subdomain name mapping

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- [x] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
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### Amplify Hosting feature

Deployments

### Is your feature request related to a problem? Please describe:

I have an app with branch auto detection on with a pattern, for example, of `foo/**`. I have the `example.com` automatic subdomain creation also set to `foo/**`. When a branch `foo/bar` is created, the corresponding subdomain becomes `foo-bar.example.com`. I would like to be able to pattern match or have some other way to have the subdomain be `bar.example.com`.

Apologies if this feature already exists as I could not find anything in the documentation.

### Describe how you'd like this feature to work

Perhaps we could have the branch detection take a regex like `foo/(?.*)` and allow the custom subdomain to use the capture group `${subdomain}`.

Start by locating the branch auto detection and automatic subdomain creation implementation; no source files, tests, or entry points are identified in the issue. Done means supporting a configured mapping where branch foo/bar produces bar.example.com, potentially through a captured pattern segment, with the behavior documented and tested.

Written by the indexing model from the issue text.