[FIX] snailmail: allow to configure timeout for snailmail

When sending a letter with a PDF containing pictures, it could happen that the default timeout of 15s is too short.

Parameter "snailmail.timeout" has been added to allow configuration of this timeout.
Also a default value of 30s has been set instead of 15s.

opw-2276125

closes odoo/odoo#53985

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>