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.