Update Validate() to use CustomAttributes

With the `CustomAttributes` now exposed to the underlying `ItemConfiguration` as a protected property (9705824), the `PropertyConfiguration.Validate()` method can now access those instead of making a direct call to `GetCustomAttributes()`. This also updates the code to use the `MemberAccessor` version of `GetValue()` instead of going directly through reflection, which should have a slight performance benefit.

Technically, this could have been done before when the dependency on `MemberAccessor` was introduced to `PropertyConfiguration` (49ffd61). As we hadn't updated `ItemConfiguration` to operate off of `CustomAttributes` yet, however, I waited to migrate this so they could operate off of the same base property (9705824).

Author: JeremyCaney

OnTopic/Mapping/Internal/PropertyConfiguration.cs

@@ -75,8 +75,8 @@ internal PropertyConfiguration(MemberAccessor memberAccessor, string? attributeP
     /// </summary>
     /// <param name="target">The target DTO to validate the current property on.</param>
     internal void Validate(object target) {
-      foreach (ValidationAttribute validator in Property.GetCustomAttributes(typeof(ValidationAttribute))) {
-        validator.Validate(Property.GetValue(target), Property.Name);
+      foreach (ValidationAttribute validator in CustomAttributes.OfType<ValidationAttribute>()) {
+        validator.Validate(MemberAccessor.GetValue(target), MemberAccessor.Name);
       }
     }