[#65619490] NatService EdgeGateway interface lookup optimisation

The NatService was calling the API to look up the Interface section for each NAT rule.

This PR introduces a cache of these lookups, so that we only lookup once per interface.

Also noticed that NatService test was broken -- the input configs were still specifying 'network' by name rather than 'network_id'. I've corrected this here, since a code improvement required highlighted the test bug.