Adding sub-domains to a domain in Amplify Apps

### 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 read the guide for [submitting questions](https://github.com/aws-amplify/amplify-hosting/blob/master/CONTRIBUTING.md).
- [X] I have removed any sensitive information from my code snippets and submission.

### App Id

d3n83m9lokqccc

### AWS Region

ap-south-1

### Amplify Hosting feature

Not Applicable

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

There is a way to add new sub-domains.
Domain management --> Manage subdomains --> Add

But there is no way to achieve this programmatically.

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

I would like to add new sub-domains programmatically through an API call or by using SDKs.

Start by determining which Amplify Hosting API or SDK surface manages domains and subdomains in ap-south-1. Done means users can add a subdomain programmatically, with the supported API or SDK behavior documented and verified.

Written by the indexing model from the issue text.