EVPN Unreachability Signaling

This document defines a new EVPN Route Type for signaling prefix unreachability information without affecting the forwarding plane. The route type reuses the Route Type 5 (IP Prefix Advertisement) field order defined for EVPN IP prefix routes, adds an Address Family octet for unambiguous IPv4/IPv6 parsing, and appends Reporter TLVs that allow aggregation of unreachability reports from multiple network vantage points.