provide framework independent version of Task.CompletedTask

src/csharp/Grpc.Auth/GoogleAuthInterceptors.cs

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

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

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

src/csharp/Grpc.Core/Utils/TaskUtils.cs

src/csharp/Grpc.IntegrationTesting/InterarrivalTimers.cs