network: Support bridge mode for zones

network: Support bridge mode for zones

This bridge mode is supposed to be used for virtual environments
to create a network zone as a bridge and have virtual machines inside
it. Other physical interfaces can also be added to the bridge.

This is very similar to the MACVTAP bridge feature but still works
when the link of any (or all) physical interfaces is down.

Fixes: #11252
Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>