Implement Powershell Stack Workload

[satvu]

Original tracking issue: Azure/azure-functions-core-tools#5322. Content copied over:

Add the PowerShell stack workload that wires the PowerShell worker into
func init, func new, and func start flows (language detection,
project scaffolding hooks, runtime resolution). Worker package itself is
tracked separately.

Definition of Done

• Stack workload nupkg builds and publishes via the workloads pipeline.
• CI pipeline (public + official/signed) wired up for the workload.
• func init --stack powershell produces a valid project.
• Stack metadata surfaces in func templates list and func --help.
• Stack manifest validated by the schema in CI.