V4/V6 radio on IP pool create

[david-crespo]

Basically redo b21f0b1 when oxidecomputer/omicron#8761 is in. Also make sure adding IPv6 ranges works.

[askfongjojo]

A related change is adding an IP Version column to the Silo > IP Pools grid.

We'll probably want to show the pool type (unicast vs multicast) there too.