allow sending trailers from server handler

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

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

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

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

src/csharp/ext/grpc_csharp_ext.c