Causali pagamento per ritenute d'acconto

This is a small bridge module that connects two Italian localization features: withholding taxes and payment reason codes (causali di pagamento).

On every withholding tax record, a new field called causale_pagamento_id is added. It is a link (many-to-one) to a payment reason code, so each withholding tax can be associated with one causale di pagamento.

In the withholding tax form, this new field appears right after the payment term field, so users can pick the payment reason code while editing or creating a withholding tax.

The module does not add menus of its own; it extends the existing withholding tax screen provided by the Italian withholding tax module. It also does not add any automatic checks, computations, emails or scheduled jobs — it only stores the link between the two records.

The module is marked as auto-install, meaning Odoo installs it automatically when both the withholding tax and payment reason modules are present. It touches the withholding.tax model and the causale.pagamento model.