Minor cleanup of subnet pool API

[bnaecker]

I'm working on wiring the subnet pools database work from #9578 to the public Nexus API from #9686. There are a couple of minor inconsistencies and tweaks we should address.

• We support a pool type on subnet pools, multicast or unicast. This makes sense for IP pools, but it seems less useful for subnet pools. We certainly don't do replication based on prefixes now and the database work all has no notion of multicast vs unicast subnets.
• Subnet pool members, the individual subnets in them, currently have all the identity metadata that most API resources have, like a name, description, etc. I think we want these to be more like IP Pool Ranges, where the only identifying information is a UUID and the CIDR block itself. You can't update these, and names don't really do anything for you, since the IP subnet is the identity of the thing.