Blue/Green Deployment Strategy

### 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

-

### AWS Region

us-east-1

### Amplify Hosting feature

Deployments

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

To us being able to control exactly when network traffic is swapped between an old and a new host during blue green deployments is one of the best features of blue green for us. Currently a build can take a few minutes and finish to finish.

As part of blue green deployments we like to be able to:
- Test before hand on the green environment before it goes live in the production system
- Being able to roll back very quickly if there is something wrong with the deployment
- Know exactly when the swap over of traffic to the new production service happens

The prior issue to this #444 covered atomic deployments allowing for instant swap over. But there is very little chance to control when this swap over occurs and test the build before it goes live naively. Would be a great feature to have!
Thanks! :smile:

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

Similar to the feature set offered by AWS CodeDeploy BlueGreen deployments.

Start by reading prior issue #444 and the AWS CodeDeploy BlueGreen feature set referenced in the request; the repository's FAQ.md and CONTRIBUTING.md provide project context. Done should let users test a green deployment, control the production traffic swap, identify when it occurs, and roll back quickly if needed.

Written by the indexing model from the issue text.