Self Manage DNS records for Route 53 domains that exist in same account

### 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

Public APIs

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

When configuring Route 53 custom domain where hosted zone exist in the same account as that of the application, Amplify automatically creates/updates/deletes the DNS records based on domain configuration.

There are use cases like [Partial (CNAME) setup](https://developers.cloudflare.com/dns/zone-setups/partial-setup/) where ability to modify underlying DNS records (after they have been configured with Amplify) where self-managing DNS records is essential.

_As on today this is only possible in case of Route 53 domains where hosted zone does exist in same account or third domains where user is expected to update the DNS records manually._

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

Ability for user to self-manage DNS records (just like in case of third party domains) should be provided by Amplify

Start by reviewing Amplify Hosting's public APIs and the Route 53 custom-domain flow described in the issue, then compare it with third-party-domain handling. Done means users can self-manage DNS records for same-account Route 53 domains without Amplify automatically changing them, including the requested partial CNAME setup.

Written by the indexing model from the issue text.