Support workloads in Microsoft.NET.Sdk

[dsplaisted]

Detailed spec is here: dotnet/designs#104. Basics are:

• Set WorkloadManifestRoot property
• Import workload manifest targets from WorkloadManifestRoot
• Add target to generate build error if there are any MissingWorkloadPack items
• Import workload .props files

[wli3]

@dsplaisted This task is your demo right?

[dsplaisted]

Yes, my demo included this (not that it was ready for check-in).

[dsplaisted]

Completed, follow-up work is tracked in #13849