Include off-link route's preference in update events

RFC 4191 supports the notion of a preference value for default routers
and more-specific routes, so update the OffLinkRouteUpdate event to
include this preference value so integrators may prioritize routes
based on a route's advertised preference value.

Note, more-specific route discovery is not supported yet, but will be in
a later change.

Updates #6172.

Test: ndp_test.TestRouterDiscovery
PiperOrigin-RevId: 380243716

0 files changed, 0 insertions, 0 deletions