Support initialization of DetectNat with static address list

[ameihm0912]

Currently DetectNat dynamically produces a side input as a materialized map based in ongoing analysis.

It should be modified so it can be optionally initialized with a static list of addresses, which will always be considered gateways by transforms that make use of NAT detection.

It would be good to just load this from GCS so a pattern like we use with identity manager could work, and the input option would be this path and the file would contain a list of addresses.