added concept of ServerCallContext, that is passed to all server-side handlers

src/csharp/Grpc.Core.Tests/ClientServerTest.cs

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

src/csharp/Grpc.Core/Internal/ServerCallHandler.cs

src/csharp/Grpc.Core/ServerCallContext.cs

src/csharp/Grpc.Core/ServerMethods.cs

src/csharp/Grpc.Examples/MathGrpc.cs

src/csharp/Grpc.Examples/MathServiceImpl.cs

src/csharp/Grpc.IntegrationTesting/TestServiceGrpc.cs

src/csharp/Grpc.IntegrationTesting/TestServiceImpl.cs