update C# to the new C core auth API

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

src/csharp/Grpc.Core/CallCredentials.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelCredentials.cs

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

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

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

...csharp/Grpc.Core/Internal/ChannelCredentialsSafeHandle.cs

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

...arp/Grpc.Core/Internal/NativeMetadataCredentialsPlugin.cs

src/csharp/ext/grpc_csharp_ext.c