Backport of the odoo/odoo#208082, adding the possibility to synchronize invoice and bill status at once.

The endpoint `ListaMesajeFactura` returns a list of messages to process.
Those messages can:
- Return an invoice status update with a success or an error;
- Return a new bill from another vendor already validated by the
authority.

The messages contain a message ID to recover and download the official data from the ANAF server: the invoice/bill XML and the signature.

(documentation)[https://mfinante.gov.ro/static/10/eFactura/prezentare%20api%20efactura.pdf]

When synchronizing, we retrieve the new messages from the last 24 hours and update the database content accordingly:
- in case of invoice status update, update the relevant invoice/bill status and download the signature in case of success (note that this was already done before, we only call the relevant methods to do so);
- in case of a new vendor bill, we download the bill XML and signature, create a new in_invoice move and fill it up with the XML content. In case a bill with the same index is already in the database, we do nothing to avoid duplicate. If a similar bill (same date and amount) is found but not yet validated, we only validate it.

The synchronization can be triggered manually through the option in the list view and it is automatically triggered every 4 hours with a CRON.

Lastly, the created bill journal can be set up in the Accounting settings so that users can specify their favorite journals for automated ANAF bills.

task-4891081

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