Optionally prevent destroying the egress static-ip.

Once a particular static IP has been de-allocated (via a terraform destroy), it's highly unlikely that you'll get the same one back if you create a new one. If that IP has already been embedded in related systems, it will probably be painful to get it changed. Therefore it would be beneficial to have a protect_static_egress_ip (or similar) module option that would protect the static IP resource with a prevent_destroy clause.