[FIX] account_edi_proxy_client, l10n_it_edi, l10n_it_edi_sdicoop: Fixing Base64 issues

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()

* `addons/l10n_it_edi/models/account_invoice.py`
  AccountMove._prepare_fatturapa_export_values()

* `addons/l10n_it_edi_sdicoop/models/account_edi_format.py`
  AccountEdiFormat._l10n_it_post_invoices_step_1()

Part-of: odoo/odoo#77968