Want Nexus API for assigning Floating IPs to external DNS servers

Part of #8945. We want to let operators assign IP Pools to specific services, e.g, NTP or Nexus. But external DNS works a little differently, because the IPs must be known so they can be added the upstream DNS server's glue records. One way to do this is let operators create / assign specific Floating IPs for external DNS, from a dedicated IP Pool.

Note we already use Floating IPs for external DNS today. A lot of the internal machinery here is already done. The only new bits are an API for the operator to use, and the mechanisms for correctly reading this during blueprint planning to generate new zones with them.

Start by reading the parent issue #8945 and locating the existing Floating IP machinery used for external DNS. Trace how blueprint planning reads that state and generates zones. Done means operators have an API to assign specific Floating IPs from a dedicated pool and planning uses those assignments for new external-DNS zones.