[FIX] account: add hook to get payment_mthod_code

Related to https://github.com/odoo/enterprise/pull/32020


Add a hook to return the account payment method code.

opw-2985984

closes odoo/odoo#101904

Related: odoo/enterprise#32020
Signed-off-by: Nicolas Viseur (vin) <vin@odoo.com>