Separation of Prod and Non-Prod accounts for Amplify Console when using CodeCommit

**Is your feature request related to a problem? Please describe.**

Per AWS best practices, I'd like to keep my non-prod and prod environments in separate accounts with varying degrees of IAM access based on least-priviledges. I am using CodeCommit as a repository for my application code. The Amplify Console requires a connection to a specific CodeCommit branch within the same account. CodeCommit does not allow me to initiate and accept pull requests across accounts, so I have no way to maintain my master code branch in one account, then promote it to production via pull request to my prod account to trigger a build and deploy.

**Describe the solution you'd like**

I see two possible resolutions to this problem:
1) Allow Amplify Console access to CodeCommit resources in separate accounts, or
2) Allow cross-account / cross-repo pull requests in CodeCommit

No repository files, tests, or entry points are named. Start by reviewing the Amplify Console and CodeCommit account and branch integration described in the issue, then determine which proposed cross-account capability is in scope; done means a supported workflow is defined for separating non-prod and prod accounts.

Written by the indexing model from the issue text.