Purchase Tier Validation

This module brings the tier validation approval process to purchase orders. Instead of confirming a purchase order directly, an order can be routed through one or more review steps defined in the tier validation configuration, and it only becomes confirmable once all required reviews are done.

Where it appears in Odoo: - On the purchase orders list, a new "Needs my Review" filter shows the orders where you are listed as a reviewer and that are not yet done, cancelled or waiting for approval. - A "Validated" filter shows the purchase orders that have completed their validation and are ready to be confirmed. - The purchase order list also shows a validation status badge (pending, rejected or validated), hidden by default and colour-coded.

Business behaviour: - Purchase orders move from draft, sent or "to approve" states into purchase or approved states through the validation process. - The tier definition configuration now accepts purchase orders as a document type, so approval rules can be set up for them.

It touches the purchase.order model and the tier definition configuration model, and adds filters and a status column to the purchase order views.