Spec for Grpc.Core nuget package that works

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

src/csharp/Grpc.Core/Grpc.Core.nuspec

src/csharp/Grpc.Core/packages.config

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

src/csharp/Grpc.IntegrationTesting/packages.config