[Feature request] Support default password protection for all new branches

**Is your feature request related to a problem? Please describe.**
When using amplify preview to deploy code on each pr, it creates a url that is not password protected by default. The only way is to go to access control and add it manually each time.

**Describe the solution you'd like**
In access control settings, I would like to be able to set a default password for all new / existing branches, except for those I explicitly exclude (namely, the production branch)

Start by reviewing the access control settings and the Amplify preview branch deployment flow. Define how a default password applies to new and existing branches, how explicit exclusions such as production are configured, and how completion will be verified across those cases.

Written by the indexing model from the issue text.