Feature request: Allow auto-subdomain creation when root domain is a subdomain

**Is your feature request related to a problem? Please describe.**
In _Domain management_ there is usually a option available called _Subdomain auto-detection_ as seen below:


_(Source: [AWS blog](https://aws.amazon.com/blogs/mobile/automatically-create-and-delete-custom-sub-domains-for-your-branch-deployments-with-amplify-console/))_

This feature is incredibly useful, but when my _root domain_ is a subdomain already (e.g. `https://previews.mealicious.app`) then this auto-detection checkbox doesn't show.

**Describe the solution you'd like**
Please allow enabling _Subdomain auto-detection_ when the root domain is a subdomain.

Start in the Domain management implementation and trace how the Subdomain auto-detection option is currently shown for root domains. Reproduce the case where the root domain is a subdomain, then verify that the option can be enabled and that automatic subdomain handling works correctly for that configuration.

Written by the indexing model from the issue text.