[ADD] l10n_ar: Be able to validate the cbu number no matter the stdnum version

If avaliable will use the stdnum.ar.cbu.validate functionality, if not
we defined our own validate method.

closes odoo/odoo#40998

X-original-commit: 9c389525
Signed-off-by: Josse Colpaert <jco@openerp.com>