Extend IP Pools to handle multicast address space

[zeeshanlakhani]

As per RFD 488:

Assignment of multicast address spaces will be tied to specific IP pools, providing access to IP pools on a silo-by-silo basis for more fine-grained control over multicast address assignment. Users within a silo can then assign multiple multicast IP addresses to instances from a pool or our mechanism for tracking membership can associate IP addresses to a specific pool. The rack network data plane would ensure that any traffic destined to that group is delivered to all instances in the set.

The current thinking is to assign IP addresses from pools that are multicast-specific and tag these addresses specifically throughout their use in Omicron. We'd need some validation and checking to confirm that none of the queries that use IP addresses attempt to pull from multicast ranges for a SNAT/ephemeral/floating IP. And, we'd make sure these pools don't include unicast addresses.

Notes:

• We shouldn't have to touch transit_ips in the API, which currently don't allow for mcast address ranges.