webapp:Setting runtime stack for web app

## Description of the new feature
When you manually create a new web app in Azure, one of the selections is “Runtime stack”, which I can set to “.NET Core 2.2”.
But when I create a new web app via a powershell script, or even try to set its settings after the fact, there is no parameter for this. Going into it in Azure after it’s created, I see it has defaulted to “.NET” with a framework version in a separate selection. If I output the web app itself or its SiteConfig in powershell, I see nothing that seems to indicate the “Stack” selection.

## Proposed implementation details (optional)
Add the Web App (App Service) 'Stack' property to both the output in powershell as well as an optional parameter in both New-AzWebApp and Set-AzWebApp.