Register cart, order and combination extra fields in demoextrafield

[jolelievre]

Description

Companion of PrestaShop/PrestaShop#42538 (extra properties: non-conventional entity naming, default value typing, cart entity). Adds one example per newly supported entity and doubles as the manual-test vehicle for the core PR:

• cart / delivery_note (COMMON — the cart is a COMMON-only entity, it has no cart_lang/cart_shop base table): seeded once by actionCartSave, displayed on the checkout summary via displayCheckoutSummaryTop reading {$cart.extra_properties.demoextrafield.delivery_note}, and copied onto the order at actionValidateOrder — the intended pattern for cart values that must survive checkout.
• order / delivery_note (COMMON), registered with the natural entity name order (the core resolves the orders table and id_order primary key), displayed on the customer's order detail page (displayOrderDetail). Deliberately no order-grid placement (id-first pagination limitation, [Extra Properties] Grid columns are silently skipped on grids using id-first pagination (order grid) PrestaShop#42536).
• combination / ean_verified (COMMON, exposed on the combinations Admin API list) and combination / restock_note (SHOP), registered as combination (resolved to product_attribute / id_product_attribute).
• The customer extra_json field becomes the JSON showcase: the module writes a PHP structure (the core encodes it for storage) and the shared template iterates the decoded structure reads return.
• README refreshed: full entity inventory, the snake_case extra_properties Smarty key (extraProperties is Admin-API-JSON-only), the COMMON-only cart caveat.

Requires a core build containing PrestaShop/PrestaShop#42538.

How to test

1. Install the module on a 9.2 build with the core PR: the registry (Advanced Parameters > Extra Properties) lists the cart, order and combination entities.
2. Add a product to the cart, open the checkout: the seeded delivery note shows above the cart summary. Place the order: the note appears on the order detail page in the customer account.
3. On the my-account page, the customer JSON field renders as an iterated structure, not a raw string.