Implement NAT for IPv6

[bnaecker]

Converting this TODO to an issue to keep track of it:

omicron/nexus/src/app/instance_network.rs

Line 481 in 9e9e60c

// TODO: implement handling of v6 nat.

[bnaecker]

#8758 implements handling of the "Dendrite half" of NAT for IPv6. It tracks NAT entries we need to send to Dendrite for IPv6 addresses so that it knows how to encap and deliver packets for an external IPv6 addresses used by instances or our own services to the hosting sled. It does not implement the "OPTE half" of IPv6 NAT -- you still can't assign an IPv6 address to any OPTE port. That should be fairly straightforward, but there are outstanding design questions about how we actually want IPv6 addressing to work in the product. I'll make follow-up issues for plumbing this through to OPTE once we settle those questions.