Enable PR previews by default with branch auto detection

**Is your feature request related to a problem? Please describe.**
Before a release, we usually create a release branch which follows the naming convention "version-X-Y-Z", which makes it easy to enable branch auto detection in the Amplify Console with a simple pattern like "version-*". That's working great, awesome feature! However, code usually need to be merged into the release branch during the final days of testing, so PR's are opened against the release branch. The problem is that the PR's created against the release branches are not automatically built (we're using Amplify as a CI tool in this case) since the PR preview feature is not enabled by default, so we have to enter the console and enable previews for the automatically created environment each time a new release branch is pushed.

**Describe the solution you'd like**
I'd love an option to enable PR previews by default for auto detected branches, either in this configuration (something like a toggle for "enable PR previews for auto detected branches"):

...or in the settings for previews (perhaps each added pattern could get a row at the bottom for more granular control):

Thank you for a great product so far!

Start in the Amplify Console configuration for auto-detected branches and the preview settings shown in the issue. Trace how a release branch matching version-* is created and whether PR previews are enabled for its environment. Done means an option or per-pattern setting enables PR previews automatically for newly detected branches and PRs against them are built.

Written by the indexing model from the issue text.