Implement Powershell worker workload for vnext CLI

[satvu]

Original issue being tracked at Azure/azure-functions-core-tools#5321. Content copied over:

Add a PowerShell worker workload so func can host PowerShell function
apps under the v5 workload model. Covers the worker package + manifest
only; templates and the stack workload are tracked separately.

Definition of Done

• PowerShell worker workload nupkg builds and publishes via the
  workloads pipeline.
• CI pipeline (public + official/signed) wired up for the workload.
• func workload install powershell succeeds on Windows, macOS, and
  Linux.
• func start runs a hello-world PowerShell function app end to end.
• Workload manifest validated by the schema in CI.