Required improvements for VMR validation in repo PRs

[NikolaMilosavljevic]

VMR validation in repo PRs was enabled with introduction of templates and scripts in: dotnet/dotnet#548 and dotnet/arcade#15840

This issue tracks the remaining work:

• Allow repo owners to specify scope of VMR validation, i.e. just one SB leg, or only Windows legs, see Enable VMR validation in repo PRs arcade#15840 (comment)
• Implement flow/sync of additional repos, i.e. nuget.client should be synced when sdk gets synced
• Evaluate if vmr-sync scripts should use arcade telemetry https://github.com/dotnet/arcade/blob/main/Documentation/Projects/DevOps/CI/Telemetry-Guidance.md. They are in the excluded list currently.