Uplinkd support for link-local only interfaces

[rcgoodfellow]

Currently uplinkd takes an interface name and an ip address through SMF parameters to set up uplinks. However, there is also a need for uplinks that only have an ipv6 link local address. In that case the control plane does not know what the address will be, only that an addrconf address needs to be created. This is primarily to support BGP unnumbered routing.

[Nieuwejaar]

This should be fairly straightforward on the dendrite side. We just need to add a way to indicate in the SMF properties that a link-local address is needed. We can either add "link-local" to the list of IP addresses in the existing property, or we can add a new property. Adding to the existing property feels the cleanest, since we can get all needed addresses in a single operation.

The part I don't have a handle on is how omicron is supposed to know how/when to set that property. There is nothing in the HostPortConfig::UplinkAddressConfig to track that - it just includes specific addresses.

[rcgoodfellow]

There is nothing in the HostPortConfig::UplinkAddressConfig to track that - it just includes specific addresses.

This is being made into an Option<IpAddr>

https://github.com/oxidecomputer/omicron/blob/d4fa7127d8be2186973c4831d67268bf4d6ad2d9/common/src/api/internal/shared/rack_init/v2.rs#L123

[internet-diglett]

We also render None as link-local when it gets stringified.