SDK should emit some build artifact around it's own version

@dsplaisted / @baronfel we discussed this back in the spring, but I couldn't find an issue.

Today in https://github.com/microsoft/component-detection we try to deduce the .NET SDK versions used to build an application based on a combination of source (global.json), intermediate artifact (project.assets.json), and environment (calling `dotnet --version`).

It would be better if there was some explicit build artifact that listed out components involved in the build.

It's not SBOM, since that needs to get the information from somewhere, this would be the thing feeding into SBOM generation.