Add support for secondary IP addresses

[ipspace]

It would be nice to have support for secondary IP addresses on devices that can do that with little hassle. The feature should be optional (gated with features.initial.secondary flag and implemented as secondary list of IPv4/IPv6 addresses (or prefixes).

The list of IPv4/IPv6 addresses should also accept named prefixes but not pools. Prefixes without the host part would be adjusted with node ID (and would have to be large enough for the node ID to fit). Trying to figure out what address allocation mechanism to use on a secondary subnet is not something I want to go into at this moment ;)

Any thoughts @ssasso @DanPartelly @jbemmel @steffann @ddutt @nmodena @ubaumann @remilocherer @petercrocker

[jbemmel]

As a transition mechanism secondary is probably better, but eventually I think we'd want to aim for:

nodes:
  r1:
    ipv4: [ 1, 2 ]
    ipv6: [ 1, 2 ]

where ipv4[0] would be the (implied) primary address.

Each device could then have a property features.initial.max_ip_addresses, default=1 if not specified

[DanPartelly]

Sounds nice.