Allow for automatic workload installation by specifying required workloads in `global.json`

[just-ero]

Problem Description

Currently, it is not possible to automate the installation of a required workload for projects. It would be good to not have to remind contributors to a project to run dotnet workload install <workload> before they can build.

The feature would additionally be useful for CI jobs, which would no longer have to manually install required workloads.

Proposed Solution

An entry in the global.json file specifying the required workloads (and, potentially, their version) which the SDK then picks up on and installs before a restore/build would be ideal:

{
  "sdk": {
    "version": "8.0.404"
  },
  "workloads": ["wasi-experimental"]
}

or:

{
  "sdk": {
    "version": "8.0.404"
  },
  "workloads": {
    "wasi-experimental": {
      "version": "8.0.404"
    }
  }
}

[marcpopMSFT]

@baronfel we were just talking about this. No plans at the moment for this but good to keep in mind.