Add top-level global.json to pin dotnet SDK version.
Add top-level global.json to pin dotnet SDK version.

googleapis/google-api-dotnet-client,Duikmeester/google-api-dotnet-client,googleapis/google-api-dotnet-client,Duikmeester/google-api-dotnet-client,Duikmeester/google-api-dotnet-client,jskeet/google-api-dotnet-client,chrisdunelm/google-api-dotnet-client,googleapis/google-api-dotnet-client,jskeet/google-api-dotnet-client,jskeet/google-api-dotnet-client,chrisdunelm/google-api-dotnet-client,Duikmeester/google-api-dotnet-client,chrisdunelm/google-api-dotnet-client,chrisdunelm/google-api-dotnet-client