Account Payment Order Tier Validation

This module plugs Payment Orders into Odoo's tier validation system, so a payment order can require one or more approvals before it moves forward.

• The approval flow applies to account.payment.order: a payment order starts in draft and, once it is sent for validation, it can only move to the open state after the configured reviewers approve it.
• Reviewers are defined through the standard tier validation configuration, which now also offers Payment Orders as a selectable document type.
• On the Payment Orders list you get two new filters: "Needs my Review", showing the payment orders waiting for your approval, and "Validated", showing payment orders that have already been approved and are ready to be confirmed.
• The module only adds the validation layer; it does not change amounts, bank details or payment generation itself.

It touches the account.payment.order model and the tier definition configuration model, and extends the existing Payment Orders search view.