Introduce yaml variables file for shared tooling into eng/common

The VMR introduces possible skew between what is built officially and what the repos test against. The VMR builds using https://github.com/dotnet/dotnet/blob/release/10.0.1xx/eng/pipelines/templates/variables/vmr-build.yml#L260-L293. However, repos like runtime and SDK may build using different images. These builds aren't used, but they have required testing. We need to know that the specific version macos used for building doesn't introduce problems. Runtime and VMR skew can jeopardize that validation.

Proposal is to put something in eng/common which can represent the latest set of images (docker, linux, windows, and mac) for public and internal builds. Repos that want to opt in to align with the VMR would use this file.