Add hook to MSBuild for importing target after Directory.Build.props file

[dsplaisted]

For the Artifacts Output Format, the .NET SDK needs to add MSBuild logic to be evaluated immediately after any Directory.Build.props file is imported. In order to do this, it is currently importing Directory.Build.props itself and disabling the import from the common targets: https://github.com/dotnet/sdk/blob/d1773f68a8bbdaa7e2228a7f2f8f0ac341dfd65b/src/Tasks/Microsoft.NET.Build.Tasks/sdk/Sdk.props#L37-L58

Instead of repeating the MSBuild logic in the SDK, we could add a hook to MSBuild to allow the SDK to specify a file to be imported directly after the Directory.Build.props file. Probably this would involve adding the following line here:

 <Import Project="$(AfterDirectoryBuildPropsImport)" Condition="$(AfterDirectoryBuildPropsImport) != ''" />

[dsplaisted]

@Forgind, could you look at doing this for VS 17.7, and then once it ships updating the .NET SDK logic to take advantage of it?

FYI @marcpopMSFT @donJoseLuis