[16.0] fetchmail_attach_from_folder: Add option to fetch without reading

[ikus060]

Is your feature request related to a problem?
For my usecase, I want odoo to process emails from a GMail folder, but I do now want Odoo to maked them as read. Moslty, because we still want to users to continue processing their emails as usual with GMail web interface instead of Odoo, while having a copy of those emails in Odoo.

Current implementation make use of "(RFC822)" fetch command which mark email as read on the server.

Describe the solution you'd like
As a workarround, I changed the implementation to replace fetch "(RFC822)" by fetch "(BODY.PEEK[])" which behave properly be fetching the emails but without changing the seen flag on the server.

Describe alternatives you've considered
N/A

Additional context
I would recommand to change the current implementation to make use of "(BODY.PEEK[])" all the time instead of "(RFC822)"