lib: add new nexthop's attributes seg6 (step3)

lib: add new nexthop's attributes seg6 (step3)

This commit add new nexthop's addional object for SRv6
routing about seg6 route. Before this commit,
we can add MPLS info as additional object on nexthop.
This commit make it add more support about seg6 routes.

seg6 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>