lib: Private api for nexthop_group manipulation

lib: Private api for nexthop_group manipulation

Add a file that exposes functions which modify nexthop groups.
Nexthop groups are techincally immutable but there are a
few special cases where we need direct access to add/remove
nexthops after the group has been made. This file provides a
way to expose those functions in a way that makes it clear
this is a private/hidden api.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>