Want Nexus external API support for dual-stack guest networking

[bnaecker]

Users can specify the details for an instance's NICs in the InstanceCreateParams. That includes the NICs with their private addresses and any number of external IP addresses. These types needs to be extended to allow dual-stack guest networking.

The InstanceNetworkInterfaceAttachment enum lets users ask for no NIC at all (not very useful), a "default" NIC, or any number of custom NICs. We should clarify or modify what the "default" NIC means with regards to dual-stack. IPv4-only is backwards-compatible, but we might want to have the new default be dual-stack. We could also add a new variant DefaultDualStack, if that's easier.

The InstanceNetworkInterfaceCreate type also needs to be updated to support dual-stack. Right now that has a single, optional ip field. We should extend that to an enum letting users pick:

• no IP at all
• auto-assign an IP from the VPC Subnet
• use an explicit IP from the VPC Subnet

Then we need to keep the existing ip field and add a new ipv6 field. I'm not sure if we also need to change the transit_ips field.

[bnaecker]

We also need to add dual-stack support to the InstanceNetworkInterface type we use to represent NICs in the public API.

[bnaecker]

We should probably restrict the transit_ips that can be set on any NIC to be of the same version as the IP stack of the NIC itself. That is, you can only add an IPv4 transit IP network if the NIC has an IPv4 address (is IPv4-only or dual-stack).