Create Required attribute and tag properties

SabreTools.Models/RequiredAttribute.cs

@@ -0,0 +1,9 @@
+namespace SabreTools.Models
+{
+    /// <summary>
+    /// Marks a property as required on write
+    /// </summary>
+    /// <remarks>TODO: Use reflection to determine required fields on write</remarks>
+    [System.AttributeUsage(System.AttributeTargets.Property)]
+    public class RequiredAttribute : System.Attribute { }
+}