Add azure environments detection methods to Microsoft.Extensions.Azure

[pakrym]

Something like:

public static class HostEnvironmentEnvExtensions
    {
        public static bool IsAzureWebApp(this IHostEnvironment hostEnvironment)
        {
            return !string.IsNullOrEmpty(Environment.GetEnvironmentVariable("WEBSITE_SITE_NAME"));
        }
    }

To be used in the following way:

            if (WebHostEnvironment.IsAzureWebApp())
            {
                services
                    .AddDataProtection()
                    .PersistKeysToAzureBlobStorage(new Uri("https://msidataprotection.blob.core.windows.net/dataprotection/keys"), new DefaultAzureCredential());
            }