update csproj project to Google.Apis.Auth 0.15.0

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Auth/packages.config

...ationTesting.Client/Grpc.IntegrationTesting.Client.csproj

src/csharp/Grpc.IntegrationTesting.Client/packages.config

...ationTesting.Server/Grpc.IntegrationTesting.Server.csproj

src/csharp/Grpc.IntegrationTesting.Server/packages.config

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

src/csharp/Grpc.IntegrationTesting/packages.config