Migrate C# to the new auth API

src/csharp/Grpc.Auth/AuthInterceptors.cs

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

src/csharp/Grpc.Auth/GrpcCredentials.cs

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

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

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

src/csharp/Grpc.Core/CallOptions.cs

src/csharp/Grpc.Core/ClientBase.cs

src/csharp/Grpc.Core/Credentials.cs

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

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

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

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

src/csharp/Grpc.IntegrationTesting.Client/packages.config

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

src/csharp/Grpc.IntegrationTesting.Server/packages.config

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

...csharp/Grpc.IntegrationTesting/MetadataCredentialsTest.cs

src/csharp/ext/grpc_csharp_ext.c