Implement Powershell templates workload

[satvu]

Original issue tracked at Azure/azure-functions-core-tools#5323. Content copied over:

Add the PowerShell templates workload covering the basic triggers (HTTP,
timer, queue, blob) so func new produces working PowerShell functions
under the v5 workload model.

Definition of Done

• Templates workload nupkg builds and publishes via the workloads
  pipeline.
• CI pipeline (public + official/signed) wired up for the workload.
• func new surfaces HTTP, timer, queue, and blob templates for
  PowerShell.
• Each generated template builds and runs locally via func start.
• Templates manifest validated by the schema in CI.