Allow labelling of Static Egress IPs

As an implementation of https://gitlab.developers.cam.ac.uk/uis/devops/admin/-/issues/223#note_226657, allow labelling of static egress IPs to indicate which resource they are targeting which will require a firewall exception.

These labels should be in a standard format so that we can run a report over all static IPs registered to pick determine what IP requires what firewall exception.