Added endpoint for registering a user locally after successful logon from Auth0

YABA.Service/Interfaces/IUserService.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using YABA.Service.DTO;
+
+namespace YABA.Service.Interfaces
+{
+    public interface IUserService
+    {
+        public bool IsUserRegistered(string authProviderId);
+        public UserDTO RegisterUser(string authProviderId);
+    }
+}