Programatically create PR preview

### Before opening, please confirm:

- [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+).
- [X] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Deployments, Frontend builds, Public APIs, Web previews

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

Due to lack of AWS Amplify functionality we can't effectively use PR Preview feature:
- #3959
- #3960
- Hard limit of 50 branches per app [^1]

[^1]: https://docs.aws.amazon.com/amplify/latest/userguide/quotas-chapter.html

As result, we want to create PR previews using AWS CLI, but after testing we found that [`aws amplify create-branch`](https://docs.aws.amazon.com/cli/latest/reference/amplify/create-branch.html) call can't replicate behaviour of automatically created PR previews.

Even though it's possible to set `--stage PULL_REQUEST` resulting branch missing `destinationBranch` and `sourceBranch` properties and as result it's not being treated as PR branch by Amplify Hosting. It seems like those properties are not available in Public APIs.

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

AWS Amplify allows me to create PR preview branch via CLI, something like:
```
aws amplify create-branch --pr-id 3333 --enable-automatic-build
```

Which in turn will replicate behaviour of automatically generated PR preview, namely:
1. Create PR preview branch
2. Add status check to PR in git provider
3. Publish PR preview URL in PR comments