Add features to MSBuild SDK Resolver interface to support optional SDK workloads

[dsplaisted]

Workloads overview spec

Implementation details: Workload resolution in MSBuild

We will update the SDK Resolver interface so that the result can:

• Return any number of SDK paths (zero, one, or many). It will be possible to return a successful result that returns no SDK paths.
• Return MSBuild items and properties to add to the evaluation result