[Powershell] Allow a list of modules to be installed

[ross-p-smith]

It would be nice if you could define a list of modules that should be installed after PowerShell was installed.

As an example, we could define

"features": {
    "ghcr.io/devcontainers/features/powershell:1": {
            "module": ["Az.Resources"]
    }
}

This would prevent having to add a postCreateCommand

Install-Module -Name Az.Resources -AllowClobber -Scope CurrentUser