added C# interop server

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

src/csharp/Grpc.Core/Server.cs

src/csharp/Grpc.IntegrationTesting.Client/.gitignore

...ationTesting.Client/Grpc.IntegrationTesting.Client.csproj

src/csharp/Grpc.IntegrationTesting.Client/Program.cs

...Grpc.IntegrationTesting.Client/Properties/AssemblyInfo.cs

src/csharp/Grpc.IntegrationTesting.Server/.gitignore

...ationTesting.Server/Grpc.IntegrationTesting.Server.csproj

src/csharp/Grpc.IntegrationTesting.Server/Program.cs

...Grpc.IntegrationTesting.Server/Properties/AssemblyInfo.cs

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

...csharp/Grpc.IntegrationTesting/InteropClientServerTest.cs

src/csharp/Grpc.IntegrationTesting/InteropServer.cs

src/csharp/Grpc.sln