[FIX] l10n_it_edi : invoice template node order

The RappresentanteFiscale and CessionarioCommittente nodes must be in this order in the invoice xml. The RappresentanteFiscale node is only added if the company has a tax representative configured. In that case, before this fix, submitting an invoice to the SDI (Italian tax agency) would not go through and generate an error :
The invoice has been refused by the Exchange System
File non conforme al formato : Invalid content was found starting with element 'RappresentanteFiscale'. One of '{TerzoIntermediarioOSoggettoEmittente, SoggettoEmittente}' is expected.

How to reproduce :
- Install l10n_it_edi_sdicoop
- Configure a tax representative on the company
- Go to accounting settings > Electronic Document Invoicing and choose the test mode + check the “Allow odoo…” checkbox.
- Create a new invoice and confirm
- Click on “Send now”
- Click again on “Send now” until there is an update and the error is displayed.

closes odoo/odoo#100129

Task: opw-2952098
Signed-off-by: Josse Colpaert <jco@odoo.com>