External IPs should store the type of their parent as an enum

[FelixMcFelix]

Today, we're differentiating the type of an external IP's parent (service or instance) using the is_service flag. The communication probe PR (#4585) adds a new class of entity which an IP can be owned and used by, and stores this using the is_probe flag.

When we add additional networking objects which could own one or more external IPs (e.g., load balancers, Internet gateways) we will want to represent the type of an IP's current owner using a single enum.