Added whitelist auth provider to lib

Author: lauren-mills

lib/csharp/microsoft.bot.builder.skills/Microsoft.Bot.Builder.Skills/Auth/WhitelistAuthenticationProvider.cs

@@ -0,0 +1,24 @@
+using System.Collections.Generic;
+using Microsoft.Extensions.Configuration;
+
+namespace Microsoft.Bot.Builder.Skills.Auth
+{
+    /// <summary>
+    /// Loads the apps whitelist from settings.
+    /// </summary>
+    public class WhitelistAuthenticationProvider : IWhitelistAuthenticationProvider
+    {
+        public WhitelistAuthenticationProvider(IConfiguration configuration, string whitelistProperty = "skillAuthenticationWhitelist")
+        {
+            // skillAuthenticationWhitelist is the setting in appsettings.json file
+            // that conists of the list of parent bot ids that are allowed to access the skill
+            // to add a new parent bot simply go to the skillAuthenticationWhitelist and add
+            // the parent bot's microsoft app id to the list
+            var section = configuration.GetSection(whitelistProperty);
+            var appsList = section.Get<string[]>();
+            AppsWhitelist = appsList != null ? new HashSet<string>(appsList) : null;
+        }
+
+        public HashSet<string> AppsWhitelist { get; }
+    }
+}