[FIX] l10n_sa_edi: add support for global discounts

        This commit does two things:

        1) Global Discounts: When creating an invoice with a negative line, such as a Tax Invoice with global discount, we process the line as a document level allowance as required by ZATCA, and remove it from the lines sent in the UBL. This allows us to provide support for global discounts, such as those applied by promotion programs

        2) Add support for down-payment document type in _l10n_sa_get_invoice_type method: 386

closes odoo/odoo#154437

X-original-commit: a5aa6c9a
Signed-off-by: Josse Colpaert <jco@odoo.com>
Signed-off-by: Mehdi Bendali Hacine (mebe) <mebe@odoo.com>