[ADD] rental_deposit: adding deposit to rental products.

[adpaw-odoo]

Introduced a new custom module that adds deposit configuration on rental products and automatically manages deposit lines on sale orders and the webshop. This allows businesses to enforce a refundable deposit whenever a rental product is added, both from the backend and the frontend.

WHY this change is being made:
Currently, when a customer rents a product there is no built-in way to collect or track a deposit amount tied to that rental. Salespersons had to manually add deposit lines every time, which is error-prone and inconsistent. This module automates that process and ensures deposit lines are always present, correctly priced, and protected from accidental edits or deletions.

Technical decisions taken:

1. Added require_deposit (Boolean) and 'deposit_amount' on 'product.template' to allow per-product deposit configuration under the Rental Prices tab.
2. Extended 'res.company' with 'deposit_product_id' and exposed it via 'res.config.settings' using 'related=' to set a global deposit product that gets added as the order line product.
3. Overrode create on sale.order.line with @api.model_create_multi to auto-add a deposit line whenever a qualifying rental product is added.
4. Implemented write and @api.ondelete overrides to keep deposit line quantity in sync and remove it when the rental line is deleted.
5. Used context keys skip_deposit_write_check and skip_deposit_delete_check to allow internal updates while blocking direct user edits on deposit lines.
6. Inherited the webshop product template to display the deposit amount and added a JS widget (RentalDepositWidget) to update the displayed amount dynamically when the quantity selector is changed by the customer.

[robodoo]