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.

updates/0.20/ver_0.229_sql.txt

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+ALTER TABLE `pp_shop_products_custom_fields` ADD COLUMN `is_required` TINYINT NOT NULL DEFAULT 1 AFTER `name`;