Supporting Flexible Algorithm Prefix SIDs in LSP Ping/Traceroute

RFC8287 defines the extensions to MPLS LSP Ping and Traceroute for Segment Routing IGP-Prefix and IGP-Adjacency Segment Identifier (SIDs) with an MPLS data plane. [I-D.ietf-lsr-flex-algo] proposes a mechanism to allow IGPs to compute constraint based path over network and use Segment Routing Prefix-SIDs to steer packets along the constraint- based paths. All Prefix-SIDs associated with the Flexible Algorithm are assigned to the same IPv4/IPv6 Prefix. Any Segment Routing network that uses Flexible Algorithm based path computation needs additional details to be carried in the FEC Stack sub-TLV for FEC validaiton. This document updates [RFC8287] by modifying IPv4 and IPv6 IGP-Prefix Segment ID FEC sub-TLVs to also include algorithm identification.