add "public" pseudo-segments for public IP addresses

[mmguero]

When enriching with netbox, source and destination network segments are populated from IP addresses.

@ee-hex-ee requested that we also put in "public" for a pseudo-segment (pseudo because it won't actually exist in netbox) for public IP addresses, so it will work in things like sankey diagrams.

Should it populate the cross-segment tag?

[mmguero]

Done. cross_segment tag is set, source.segment.name and/or destination.segment.name will be set to public when applicable.