[FIX] account_edi_proxy_client, l10n_it_edi_sdicoop: adapt config params to...

[FIX] account_edi_proxy_client, l10n_it_edi_sdicoop: adapt config params to make use of the test iap server

This commit adapts the l10n_it_edi_sdicoop and account_edi_proxy_client
modules to make use of the existing config param
'account_edi_proxy_client.demo' in order to target the correct odoo iap
server for italian edi requests.

When the config param is set to:
'prod' or False (when the config param doesn't exist): Target the
production odoo iap server (by default), which submits and collects edi from the
production channel. Though if the config parameter
'account_edi_proxy_client.edi_server_url' exists, we use the value
provided there instead.

'test': Target the test odoo iap server, which submits and collects edi
from the test channel

'demo' or anything else not above: don't send anything, and simulate the
flow of italian edi submissions in odoo

task-id 2764978

Part-of: odoo/odoo#84724