Introduce targets to filter target frameworks

[mmitche]

Relies on dotnet/msbuild#8350 to be active or not cause build errors when not enabled. This is largely still experimental and may change. Enables the ability for an invocation of an arcade build to only build a desired set of target frameworks. The primary consumer is source-build. This allows source-build to simply remove TFMs from the build that would require SBRPs without extensive changes in project files.

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/arcade/tree/main/Documentation/Validation

[tkapin]

@mmitche Can you advise on a setup to simulate and check this locally (e.g., repos to test with, etc.)? Cheers

[MichaelSimons]

To roll this out in source-build, is the idea we would enable this on a per repo basis starting at installer and then once all repos are onboarded we would enable it by default at the source-build level and remove the per repo settings?

[mmitche]

@mmitche Can you advise on a setup to simulate and check this locally (e.g., repos to test with, etc.)? Cheers

I only had a hacked up setup in source-build that was half working. This is now in the SDK, so I'm working on getting it into arcade for testing in non-SB setups.