Support optional UB and SB legs in repo PR validation

[MichaelSimons]

The Repository and VMR PR/CI validation Strategy talks about support for optional UB and SB legs in repo PR validation. This would involve syncing the PR into the VMR, building, and running tests. This could be added to the arcade templates similar to how the repo source-build leg is implemented today.

[premun]

This would be best implemented as a set of scripts/YAML templates in Arcade that the repos could take advantage of. The steps would prepare the VMR similarly to how it's done today in dotnet/sdk (see vmr-prepare.yml, vmr-pull-updates.yml).
If we only care about building the VMR, this whole can mostly be just one job template that repos could include in their pipelines.

Please also see dotnet/arcade-services#4322

Whoever works on this, feel free to reach out to us and we can help pointing you in the right direction.