expose AuthContext in C#

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

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

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

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

src/csharp/Grpc.Core/AuthContext.cs

src/csharp/Grpc.Core/AuthProperty.cs

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

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

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

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

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

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

src/csharp/Grpc.Core/Metadata.cs

src/csharp/Grpc.Core/ServerCallContext.cs

src/csharp/Grpc.IntegrationTesting/SslCredentialsTest.cs

src/csharp/ext/grpc_csharp_ext.c

src/csharp/tests.json