Define helper script for source-building VMR feature bands

[mthalman]

There are a number of steps that are needed in order to setup the configuration properly to source build feature band branches (non-1xx) in the VMR. For developers that are ok with using build assets created by Microsoft as input, we should have a helper script. This would essentially be the equivalent of what the source-build-prep script is doing, in that it downloads Microsoft-owned assets that are referenced by the VMR. This is what the CI pipeline is doing. It would just distill that into a helper script.

[mthalman]

[Triage] Need to examine what the UX will be for potentially including this logic in the existing prep script rather than defining a separate one.