set retain policy to destroy for resources created by PR deploy previews

[josefaidt]

Environment information

n/a

Describe the feature

Customers have the option to enable previews for pull requests against their connected repository. This enables folks to create a new backend for each pull request, which is treated as a branch deployment with a special "pr" identifier. When pull requests are closed or otherwise merged, resources should be destroyed.

When backends are created from PR deploy previews, resources within the stack should have their retain policy set to DESTROY, similar to sandbox deployments.

Use case

n/a

[ykethan]

Marking as feature request for further improvements.