add scripts for automated building of C# nuget on Kokoro

kokoro/release/csharp/windows/build_nuget.bat

+@rem enter repo root
+cd /d %~dp0\..\..\..\..
+
+cd csharp\src
+call build_packages.bat

kokoro/release/csharp/windows/release.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/release/csharp/windows/build_nuget.bat"
+timeout_mins: 60
+
+action {
+  define_artifacts {
+    regex: "**/*.nupkg"
+  }
+}