Feature branch support

[mthalman]

There are times when a complex feature in Image Builder and/or the common infrastructure that needs to be iterated on until completion. Each time a change is made to the eng/common folder, it gets pushed out to our other Docker repos. If there's a lot of iteration on a feature, this churn can incur heavy costs on resources to run PR builds for these changes.

A better approach is to have a feature branch in this repo that the feature is worked on. But we'd still want to have an automated way of pushing those changes from that feature branch to a designated product repo/branch such as dotnet-docker/nightly.

This issue proposes that the push-common-updates pipeline be updated to handle this scenario.