added backend for storing extra fields.

Models/RecordExtraField.cs

@@ -0,0 +1,11 @@
+namespace CarCareTracker.Models
+{
+    public class RecordExtraField
+    {
+        /// <summary>
+        /// Corresponds to int value of ImportMode enum
+        /// </summary>
+        public int Id { get; set; }
+        public List<ExtraField> ExtraFields { get; set; } = new List<ExtraField>();
+    }
+}