new feature: sieve forward plugin

[Marc]

As this page[1] describes a more often occurring problem of forwarding messages from servers that are not included in the spf records. Maybe there should be a plugin that offers this forward functionality. Something like

get the spf records of the sender
check if there is a -all
then apply the sender substitution.

https://doc.dovecot.org/configuration_manual/sieve/configuring_auto_forward_sender_address/