[17.0][ADD] fetchmail_local_mailbox

[hbrunn]

This module allows to retrieve mails from a mailbox on the local file
system.

That's useful when your Odoo instance runs on a server with a local
mailserver that delivers mails into a mailbox accessible by the user
Odoo is running as, because it allows you to fetch mails without any
credentials or TCP/connection overhead.

Usage

To use this module, you need to:

1. select server type Local mailbox in a fetchmail configuration
2. fill in the full path to the mailbox in the server's filesystem
3. in case you want to delete mails from the mailbox after processing,
   check Delete processed mails from mailbox