firewall: Add policy for forwarding between zones

Apparently this is needed for firewalld in Bookworm, when the FreedomBox is used as a router.

https://bugzilla.redhat.com/show_bug.cgi?id=2016864#c8

(Originally reported at https://discuss.freedombox.org/t/debian-12-bookworm-release-and-upgrading/2591/5.)