[SecretManagement] Have a SecretManagement extension Plaster scaffold

[TylerLeonhardt]

Summary of the new feature/enhancement

This is basically the PowerShell script version of #83

Plaster has an integration in the PowerShell extension. If we craft a Plaster template, then it appears in a Wizard inside the PowerShell extension which is really nice.

We should have a template for Secret Management extensions. This will help a ton with adoption because we can bootstrap the user.

Proposed technical implementation details (optional)

• A Plaster template with the files listed in the Script vault extension module section of @PaulHigin's blog post:
  https://devblogs.microsoft.com/powershell/secrets-management-module-vault-extensions/

• Maybe a CodeTour if we're feeling really flashy?