Add high port parameter to nat apis

Tracking issue for conversation started in https://github.com/oxidecomputer/dendrite-archived/pull/699.

In https://github.com/oxidecomputer/dendrite-archived/pull/699 it was discussed that we may have a small gap in behavior in how our nat APIs work. For a given IP, if the port range of an existing nat entry is contained within the larger range of an incoming nat entry, but does not land on the upper or lower port number, it will correctly be detected as a conflict when attempting to add it, but if you try to lookup the conflicting nat entry you will not find it because we only accept the low port number for lookups and deletions.

In https://github.com/oxidecomputer/dendrite-archived/pull/699 it was proposed that we expose the high port parameter throughout our APIs, however the subject of that PR was able to be accomplished more effectively without doing so.

Start with the discussion in PR #699, then trace the NAT API entry points for adding, looking up, and deleting entries. Determine where the high-port parameter would need to be exposed across those APIs; done means the APIs support the proposed parameter and the conflicting entry can be found through the relevant lookup behavior.