lib: add new nexthop's attributes seg6local (step1)

lib: add new nexthop's attributes seg6local (step1)

This commit is a part of #5853 works that add new nexthop's addional
object for SRv6 routing about seg6local route. Before this commit,
we can add MPLS info as additional object on nexthop.
This commit make it add more support about seg6local routes.

seg6local routes are ones of the LWT routing mechanism,
so configuration of seg6local routes is performed by
ZEBRA_ROUTE_SEND, it's same as MPLS configuration.

Real configuration implementation isn't implemented at this commit.
later commit add that. This commit add only nexthop additional object
and some misc functions.

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>