Add player authentication backend.

OpenRA.Game/Network/Session.cs

@@ -126,6 +126,9 @@ namespace OpenRA.Network
 			public bool IsInvalid { get { return State == ClientState.Invalid; } }
 			public bool IsObserver { get { return Slot == null; } }
 
+			// Linked to the online player database
+			public string Fingerprint;
+
 			public MiniYamlNode Serialize()
 			{
 				return new MiniYamlNode("Client@{0}".F(Index), FieldSaver.Save(this));