Add NuGet.config with public feed per guidance (#797)

nuget.config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="XlangDependencies" value="https://pkgs.dev.azure.com/shine-oss/xlang/_packaging/XlangDependencies/nuget/v3/index.json" />
+  </packageSources>
+</configuration>