[FIX] account_edi_proxy_client, l10n_it_edi_sdicoop: base64 + configurable url

When encoding a string or bytes with base64,
the resulting byte string must be decoded
to get the string back.

* `addons/account_edi_proxy_client/models/account_edi_proxy_user.py`
  AccountEdiProxyUser._register_proxy_user()

+ making the default server url configurable with an ir.config_param:

There is a test_mode to allow experimentation on the IAP server.
To be able to switch between the two, we need to make the
SERVER_URL configurable.

backport of https://github.com/odoo/odoo/pull/77968


but needed some fixes

opw-2703669

closes odoo/odoo#81715

Signed-off-by: William André (wan) <wan@odoo.com>