Expand/standardize usage of package@version syntax across the .NET CLI

[baronfel]

Back in #42367 we got agreement from NuGet to start moving onto this syntax because of familiarity from other ecosystems.

Per conversation earlier today, we agreed to the package@version syntax and we'll start with just the workload and matching manifest. We decided to just use the existing --version or workloadVersion in the global.json for this and look for the @ symbol.

We have three options for fetching this from nuget:

1. Download all of the available workload sets for the current SDK band and pick the highest that matches
2. Use the NuGet package tag to start this information and use the api to query the tags
3. Get NuGet to add a property bag queryable from the API

We agreed to proceed with one since the number of workload sets should be relatively

Originally posted by @marcpopMSFT in #42367