[Feature Request] Allow Deployments without the Frontend

### 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

Backend builds, Build settings, Frontend builds

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

Many of the supported platforms such as, iOS and Android do not rely on the Frontend deployments. That is why when we force the customers to add a _frontend_ tag to their build file it will lower the quality of the experience. In the issue [here](https://github.com/aws-amplify/amplify-hosting/issues/2450), the problem is resolved due to being stale. But it should work as described by default.

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

I want customers to have a backend building capability when they actually build the apps for only backend.

Review FAQ.md and issue #2450 first, then trace the build settings and backend/frontend build handling referenced by the issue. Done means customers can run a backend-only build without adding a frontend tag, with the behavior matching the documented default.

Written by the indexing model from the issue text.