Expose the Missing Workloads (not just missing Workload Packs) as MSBuild Items when workloads are missing

### Is your feature request related to a problem? Please describe.

When workloads are missing we populate a lot of `MissingWorkloadPack` items, which the SDK then translates into the specific workloads required to provide those packs via `dotnet workload install` and `dotnet workload restore` commands.

### Describe the solution you'd like

Today we have the [ShowMissingWorkloads](https://github.com/dotnet/sdk/blob/171723b0d76e60d147d259140adbfd078aa252a1/src/Tasks/Microsoft.NET.Build.Tasks/ShowMissingWorkloads.cs#L24-L25) task, but we should also be able to expose this during Restore for folks that need to get this mapping already. Specifically 1ES would like to know _after the Restore task is run_ what workloads are missing (if any).