add C# testing APIs

src/csharp/Grpc.Core.Testing/.gitignore

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.project.json

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.xproj

src/csharp/Grpc.Core.Testing/Properties/AssemblyInfo.cs

src/csharp/Grpc.Core.Testing/Settings.StyleCop

src/csharp/Grpc.Core.Testing/TestCalls.cs

src/csharp/Grpc.Core.Testing/packages.config

src/csharp/Grpc.Core.Testing/project.json

src/csharp/Grpc.Core/Properties/AssemblyInfo.cs

src/csharp/Grpc.sln

src/csharp/build_packages_dotnetcli.bat

src/csharp/build_packages_dotnetcli.sh

templates/src/csharp/Grpc.Core.Testing/project.json.template

templates/src/csharp/build_packages_dotnetcli.bat.template

templates/src/csharp/build_packages_dotnetcli.sh.template