Want Nexus API for operators to assign IP Pools for specific services

This tracks the last API updated needed for #8945. We want to let operators specify which IP Pools are assigned to Oxide, but more directly, which pools are used for which services. The three services that need external addresses today are Nexus's external API, NTP, and DNS.

Note that there are a bunch of related issues:

• #3732
• #8255
• #8191
• All the NTP stuff tracked under #2219

As suggested in some of the comments on those issues, it seems reasonable for these assignments to take the form of a "rack-level networking configuration" table. There may be other columns needed in that table, but this specific work would only need to track the IP Pools for each of the external services.

There is related work tracked in #8949 to use this information when building blueprints.

Start with #8945 and the related issues #3732, #8255, #8191, and #2219 to understand the required operator workflow; no implementation files or tests are named. Define the rack-level networking configuration API and its IP-pool assignments for Nexus's external API, NTP, and DNS, coordinating with #8949. Done means the configuration can track those service assignments for later blueprint use.