feat(commons): add DataType.List + ElementDataType companion for multi-value attributes

src/ZB.MOM.WW.ScadaBridge.Commons/Entities/Templates/TemplateAttribute.cs

@@ -25,6 +25,13 @@ public class TemplateAttribute
     /// </summary>
     public DataType DataType { get; set; }
     /// <summary>
+    /// For <see cref="Enums.DataType.List"/> attributes: the scalar type of each
+    /// element (String, Int32, Float, Double, Boolean, DateTime). Null for scalar
+    /// attributes. The element type is fixed by the base attribute and cannot be
+    /// changed on a derived template or instance override.
+    /// </summary>
+    public DataType? ElementDataType { get; set; }
+    /// <summary>
     /// Gets or sets a value indicating whether the attribute is locked from override.
     /// </summary>
     public bool IsLocked { get; set; }