Mark purchase order as fetched or viewed

Please be aware about the following when using this endpoint:

1. Please use the fields with their correct scopes in mind:
   1. fetched_by_supplier: This field is intended to used internally by the supplier in order to enable filtering which orders have already been fetched in the Get Supplier Purchase Orders endpoint. This field is meant to be used exclusively by the supplier and does not provide any feedback to our mutual customers.
   2. viewed_by_supplier: Used to show a feedback (as an order status update) for the customer who placed the order about whether the order was picked up and processed or not. At this point, the customer will expect to either receive the order on the requested delivery date or to be contacted by the supplier for any possible issues.
2. This endpoint will also set the respective timestamps fetched_by_supplier_at and viewed_by_supplier_at to the current time.
3. It is not possible to revert these fields back to false after they've been set to true, so be careful when using this endpoint.