Milestone: Create a BGP-LS model for Segment Routing extensions as per draft-gredler-idr-bgp-ls-segment-routing-extension

Segment Routing (SR) allows for a flexible definition of end-to-end
paths within link-state graphs by encoding paths as sequences of
topological sub-paths, called "segments".

The link-state routing protocols (IS-IS, OSPF and OSPFv3) have been
extended to advertise the segments. But flooding based propagation
of path segments using IGPs is limited by the perimeter of the IGP
domain. For building paths which span across IGP domains (e.g.
multiple ASes), the Border Gateway Protocol (BGP) is better suited as
its propagation perimeter is not limited like the IGPs.

This request is to extend current BGP-LS model to support extensions
to the BGP Link-state address-family to carry path segment
information via BGP