Update to support Cisco Codec External Sources using Essentials Room logic.

Adds new interface for Codecs IHasExternalSourceSwitching
Adds ability to set and clear External Sources in Cisco Spark device class
Adds ability to send sources from the source list to the Cisco Spark in the "EssentialsHuddleVtc1Room" room
[ ] still needs parsing when source is selected on the Cisco touch 10

essentials-framework/Essentials Core/PepperDashEssentialsBase/Room/EssentialsRoomBase.cs

@@ -51,7 +51,19 @@ namespace PepperDash.Essentials.Core
         /// <summary>
         /// The config name of the source list
         /// </summary>
-        public string SourceListKey { get; set; }
+		/// 
+		protected string _SourceListKey;
+        public virtual string SourceListKey {
+			get
+			{
+				return _SourceListKey;
+			}
+			set
+			{
+				_SourceListKey = value; 
+
+			}
+		}
 
         /// <summary>
         /// Timer used for informing the UIs of a shutdown