Add is_required column to pp_shop_products_custom_fields table

This commit introduces a new column `is_required` to the `pp_shop_products_custom_fields` table. The column is of type TINYINT, cannot be null, and has a default value of 1. This change is intended to enhance the product custom fields by allowing the specification of whether a field is mandatory.

templates/shop-product/product.php

@@ -526,7 +526,7 @@
       }
 
       // dodatkowe pola muszą być uzupełnione
-      $( '.custom-field input' ).each( function( index, element )
+      $( '.custom-field input[required]' ).each( function( index, element )
       {
         if ( $.trim( $( element ).val() ) == '' )
         {