Routing requests to a different branch on the same domain

**Is your feature request related to a problem? Please describe.**
I was reviewing the other feature requests in this repository and came across #331 which I'd like to propose an extension to. Whether this is another feature or can be incorporated into A/B testing behavior, I think it would be beneficial to route requests to another feature branch deployment but serve it from the same domain so if you're creating a private area of a Amplify site or maybe some logged in version of the site, you can access it but preserve the URL.

**Describe the solution you'd like**
I'd like to be able to route traffic to a specific branch, kind of how Netlify does it currently with A/B testing as mentioned in #331 but by setting a cookie or something similar in order to keep the domain the same but view the other branch under different conditions (such as a logged-in state or something similar).

If this is already possible to accomplish with amplify right now, I'd be interested to know how!

No repository files, tests, or entry points are identified. Start by reviewing related issue #331 and checking how Amplify currently handles branch deployments, custom domains, cookies, and A/B testing. Done would mean a defined, implementable approach for selecting another branch while preserving the same domain and the conditions that trigger it.

Written by the indexing model from the issue text.