Consider exposing the ValidateAssemblies targets in the sdk and provide user defaults

[ViktorHofer]

#26616 adds the ValidateAssemblies targets which hook the ValidateAssemblies task onto a project's build. These targets are currently only included in the transport package and aren't exposed via the SDK. We should consider exposing them which will require eliminating a few assumption and provide good defaults.

The ValidateAssemblies task currently relies on providing a set of left assemblies and right assemblies. We should see if we can leverage NuGet's apis to calculate matching tuples similar to how PackageValidation works today.

We might also want to provide a default transformation pattern that avoids the rooted full paths (i.e. C:\git\runtime\artifacts\bin\...\System.Text.Json.dll). To not depend on assumptions about how the a project's output path is constructed, we could just subtract the BaseOutputPath from the full path but with the caveat that other information like the Configuration is still encoded in the transformed path.

[ghost]

@dotnet/area-infrastructure-libraries a new issue has been filed in the Compatibility area, please triage