Product Customer code for account invoice

This module adds a customer code column to the invoice lines of customer documents in Odoo's Accounting app.

• On customer invoices and credit notes (sale documents), a new column appears next to the product on each invoice line, showing the product code defined for that customer in the product's customer information.
• The code is filled in automatically: it looks at the product and the invoice's customer, finds the matching customer-specific product code, and displays it. If no customer code is defined, the column stays empty.
• The column is hidden on documents that are not customer documents, such as vendor bills, so it only shows where it makes sense.
• The value is recalculated when the product, the customer, or the document type changes, so it always stays in sync.
• It works together with the product customer information module, which is where the customer codes are maintained.

It touches the invoice and invoice line records (account.move and account.move.line) and adds the column to the standard invoice form view.