Enable building of msix package (#9289)

Add support in packaging.psm1 to produce a .msix AppX package.  Update the docker image to use the new msix package type.  Update the associated yml files so AzDevOps performs the build.

## PR Context

Enable publishing PSCore6 to Microsoft Store

assets/AppxManifest.xml

@@ -0,0 +1,49 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<Package IgnorableNamespaces="uap mp rescap desktop6"
+  xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10" 
+  xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest" 
+  xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10" 
+  xmlns:uap3="http://schemas.microsoft.com/appx/manifest/uap/windows10/3" 
+  xmlns:desktop="http://schemas.microsoft.com/appx/manifest/desktop/windows10"
+  xmlns:desktop6="http://schemas.microsoft.com/appx/manifest/desktop/windows10/6"
+  xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities">
+
+  <Identity Name="Microsoft.PowerShell" Publisher="CN=Microsoft Corporation, O=Microsoft Corporation, L=Redmond, S=Washington, C=US" Version="$VERSION$" />
+
+  <Properties>
+    <DisplayName>PowerShell Core 6</DisplayName>
+    <PublisherDisplayName>Microsoft Corporation</PublisherDisplayName>
+    <Logo>assets\StoreLogo.png</Logo>
+    <desktop6:RegistryWriteVirtualization>disabled</desktop6:RegistryWriteVirtualization>
+    <desktop6:FileSystemWriteVirtualization>disabled</desktop6:FileSystemWriteVirtualization>
+  </Properties>
+
+  <Dependencies>
+    <TargetDeviceFamily Name="Windows.Universal" MinVersion="10.0.17763.0" MaxVersionTested="10.0.17763.0" />
+  </Dependencies>
+
+  <Resources>
+    <Resource Language="en-US"/>
+  </Resources>
+
+  <Applications>
+    <Application Id="App" Executable="pwsh.exe" EntryPoint="Windows.FullTrustApplication">
+      <Extensions>
+        <uap3:Extension Category="windows.appExecutionAlias" EntryPoint="Windows.FullTrustApplication" Executable="pwsh.exe">
+          <uap3:AppExecutionAlias>
+            <desktop:ExecutionAlias Alias="pwsh.exe" />
+          </uap3:AppExecutionAlias>
+        </uap3:Extension>
+      </Extensions>
+      <uap:VisualElements DisplayName="PowerShell Core 6" Description="PowerShell is an automation and configuration management platform. It consists of a cross-platform (Windows, Linux, and macOS) command-line shell and associated scripting language." BackgroundColor="transparent" Square150x150Logo="assets\Square150x150Logo.png" Square44x44Logo="assets\Square44x44Logo.png">
+      </uap:VisualElements>
+    </Application>
+  </Applications>
+
+  <Capabilities>
+    <Capability Name="internetClient" />
+    <rescap:Capability Name="runFullTrust" />
+    <rescap:Capability Name="unvirtualizedResources" />
+  </Capabilities>
+</Package>