Merge pull request #2964 from jtattermusch/refactor_auth_interceptors

src/csharp/Grpc.Auth/AuthInterceptors.cs

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Core/AsyncClientStreamingCall.cs

src/csharp/Grpc.Core/AsyncDuplexStreamingCall.cs

src/csharp/Grpc.Core/AsyncServerStreamingCall.cs

src/csharp/Grpc.Core/ChannelOptions.cs

src/csharp/Grpc.Core/ClientBase.cs

src/csharp/Grpc.Core/Method.cs

src/csharp/Grpc.IntegrationTesting/InteropClient.cs