Support for BGP unnumbered

Is your feature request related to a problem? Please describe. Support BGP unnumbered for kube-vip.

Describe the solution you'd like GoBGP supports BGP unnumbered, so this entails adding support for parsing BGP unnumbered peer(s) from the config. From there, it's minimal work to pass the values in AddPeer to GoBGP.

I am thinking of only adding support in the bgp_peers env var, to reduce parsing logic sprawl across the other BGP env vars that assume one peer, which I assume are really kept for backward compatibility.

Describe alternatives you've considered Can also add support in the other BGP env vars, but this will complicate the logic slightly. However, this makes it a bit more flexible in that users can use either approach to define an (unnumbered) peer.

I assume kube-vip is moving toward a CRD approach in the future which will improve the growing parsing logic sprawl.

Additional context I'm probably going to work on a PoC.