Add callback to filter custom field values on ShowCustomFields

In some scenarios, it's required to restrict custom field values to be
shown on the ticket display page, even if the value was set within
the ticket.

This commit introduces a callback for post-processing these values,
enhancing the flexibility to tailor displayed data.

share/html/Elements/ShowCustomFields

@@ -53,6 +53,12 @@
 % while ( my $CustomField = $CustomFields->Next ) {
 <%perl>
 my $Values = $Object->CustomFieldValues( $CustomField->Id );
+
+$m->callback( CallbackName => 'MassageCustomFieldValues',
+              CustomField => $CustomField,
+              Object => $Object, Grouping => $Grouping, Table => $Table,
+              CustomFieldValues => $Values, ARGSRef => \%ARGS);
+
 my $count = $Values->Count;
 next if $HideEmpty and not $count;
 my $CustomFieldName = $CustomField->Name;