[ADD] l10n_sa_edi_pos: Implement Saudi ZATCA invoicing standards (POS)

With the introduction of our l10n_sa_edi module we are able to send invoices to ZATCA,
in this commit we are forcing the invoicing of every PoS order in order to send a simplified
invoice for each PoS order.
add a fallback for l10n_sa_edi for invoice_payment_means
This commit is to add a fallback ('unknown') incase _l10n_sa_get_payment_means_code
returns value not considered

closes odoo/odoo#124300

Signed-off-by: Josse Colpaert <jco@odoo.com>