adjust C# generator to match the new API

src/compiler/csharp_generator.cc

...p/Grpc.Core.Tests/Internal/MetadataArraySafeHandleTest.cs

src/csharp/Grpc.Core/ClientBase.cs

src/csharp/Grpc.Examples.Tests/MathClientServerTests.cs

src/csharp/Grpc.Examples/MathGrpc.cs

src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs

src/csharp/Grpc.HealthCheck/HealthGrpc.cs

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

...csharp/Grpc.IntegrationTesting/InteropClientServerTest.cs

src/csharp/Grpc.IntegrationTesting/TestGrpc.cs

src/csharp/generate_proto_csharp.sh