Added (optional) lastUpdated parameter to constructor

Previously, the `AttributeValue` property had a `LastUpdated` property, but as it was automatically assigned, it wasn't included in the constructor. In preparation for allowing this to be set as part of the loading and import processes, however, this needs to be exposed as a writeable field, at least via the constructor (since, currently, the property itself is readonly). Arguably, this _might_ make more sense being placed after `value`; since mapped properties will be calling `enforceBusinessLogic`, however, but _not_ supplying `lastModified`, we're placing it last. Regardless, as ancillary metdata on an internal overload, the order isn't too critical here.

Author: JeremyCaney

OnTopic/Attributes/AttributeValue.cs

@@ -92,12 +92,28 @@ public AttributeValue(string key, string? value, bool isDirty = true) {
     ///   If disabled, <see cref="AttributeValueCollection"/> will not call local properties on <see cref="Topic"/> that
     ///   correspond to the <paramref name="key"/> as a means of enforcing the business logic.
     /// </param>
+    /// <param name="lastModified">
+    ///   The <see cref="DateTime"/> value that the attribute was last modified. This is intended exclusively for use when
+    ///   populating the topic graph from a persistent data store as a means of indicating the current version for each
+    ///   attribute. This is used when e.g. importing values to determine if the existing value is newer than the source value.
+    /// </param>
     /// <requires
     ///   description="The key must be specified for the key/value pair." exception="T:System.ArgumentNullException">
     ///   !String.IsNullOrWhiteSpace(key)
     /// </requires>
-    internal AttributeValue(string key, string? value, bool isDirty, bool enforceBusinessLogic) : this(key, value, isDirty) {
-      EnforceBusinessLogic = enforceBusinessLogic;
+    internal AttributeValue(
+      string key,
+      string? value,
+      bool isDirty,
+      bool enforceBusinessLogic,
+      DateTime? lastModified = null
+    ): this(
+      key,
+      value,
+      isDirty
+    ) {
+      EnforceBusinessLogic      = enforceBusinessLogic;
+      LastModified              = lastModified?? DateTime.Now;
     }
 
     /*==========================================================================================================================