Add ability to specify ICMP6 for firewall rule protocol

[rcgoodfellow]

Currently you can specify ICMP as a protocol filter for a firewall rule but not ICMP6 when adding a rule through the console. As of oxidecomputer/omicron#10101, ICMP6 is now an option and is available in the API.

[taspelund]

Corollary: to avoid ambiguity and get ahead of confusion, we should consider renaming the preexisting ICMP to ICMP4. ICMP has a different protocol number in IPv4 compared to IPv6, so I imagine users will be likely to get confused if they install a rule matching "ICMP" and it only works for v4 despite the name being just as applicable for v6. (I had this confusion and I doubt I would be alone in this)