[FIX] sale: show custom ptav in the right order

The value of custom ptavs is store on the sale order line.

Currently, they are ordered by ptav id but they are ordered by the ptav
sequence everywhere in Odoo.

Now, on the sale order line, custom ptav are ordered by ptav sequence.

opw-3234669

closes odoo/odoo#122161

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>