[QW][3959142] amount left to invoice PO

#Done by RHT on the 30/05/2024
#Reviewed by EMI on the 11/06/2024
#Review: https://pastebin.com/J3PuStxr
#911 task: https://www.odoo.com/odoo/project/5686/tasks/3959142?cids=1
#Need: Need to know what is the remaining amount to be billed on a PO
for po in self:
    po["x_studio_remaining_amount_to_invoice"] = po.amount_total + sum(inv.amount_total_signed for inv in po.invoice_ids)