Add dual-stack support to the `IncompleteNetworkInterface`

[bnaecker]

We use a helper type IncompleteNetworkInterface in a bunch of places to describe the properties we'd like an eventual NIC to have. For example, that has a field ip, which is an Option<IpAddr> -- None means we'd like to auto-assign a VPC-private address and Some(addr) means we'd like to use an explicit address.

This needs to be extended to support dual-stack NICs, allowing an IP address of one or both versions. We probably want to change the existing optional type to be a bit richer, letting us express three choices:

• No IP at all
• Auto-assign an IP
• Use an explicit IP

We can then use that new type for the existing ip field and a new ipv6 field or similar. We need to modify the constructors to enforce whatever invariants we want, such as ensuring at least one IP address of either family.

• Add enum for more fine-grained control over IP allocation for either family (none, auto, explicit)
• Add field for IPv6, modify existing field to be explicitly IPv4
• Update constructors for plus call sites, of which there are...many

[bnaecker]

Closed by #9269