[vs-insertion] Push packs to "shipping" feed (#2440)

Context: xamarin/yaml-templates#134

The .NET 6 packs will now also be pushed to a public xamarin
`net6-maui-shipping` feed during the `VS Insertion` stage, to
help with testing efforts that can't use the `dotnet6` feed.

eng/pipelines/handlers.yml

@@ -308,3 +308,8 @@ stages:
           **/signed/*.nupkg
           **/*.snupkg
           **/additional-assets.zip
+        pushToShippingFeed: true
+        nupkgArtifactName: nuget
+        nupkgArtifactPatterns: |
+          **/signed/*.nupkg
+        msiNupkgArtifactName: vs-msi-nugets