implement C# wrapping for plugin API

src/csharp/Grpc.Core/Credentials.cs

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

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

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

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

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

src/csharp/ext/grpc_csharp_ext.c