Implement auto-creation of staging branch

[iameli]

• Upon an upstream project cutting a new semver release (make release or whatever), a new CI action gets kicked off
• This CI action generates a new "version manifest" file based on the latest semver releases of all of the upstream projects listed in manifest.yaml
• This version manifest then gets committed onto a new branch off of main named staging. This branch is always force-pushed, so there's just one commit to merge when we want to cut a release.
• It should also open a PR, if one does not already exist, from staging to main.
• When we want to cut a new Catalyst release, we simply merge that PR and CI takes care of publishing new images.