add internal package feed (#7951)

Author: brettfo

NuGet.config

@@ -7,6 +7,7 @@
   <packageSources>
     <clear />
     <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
+    <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="fsharp-daily" value="https://www.myget.org/F/fsharp-daily/api/v3/index.json" />
     <add key="roslyn-master-nightly" value="https://dotnet.myget.org/F/roslyn-master-nightly/api/v3/index.json" />