[18.0][ADD] pos_automatic_validation: Add to 18.0

[nguyenminhchien]

This module was initially part of the Foodcoop set of addons. We're now migrating it to 18.0 and taking this occasion to contribute it to OCA.

Key Features

• Adds a new field “Automatic Validation” on each POS Payment Method.
• Automatically validates the order once it’s fully paid using eligible payment methods.
• Works only for non-terminal payments (e.g., cash, custom wallets).
• Deactivates Back and Validate buttons during automatic validation to simplify the user interface.