Implement relay IPv6 extends with proposed protover
Make relays with IPv6 ORPorts start attempting to connect to IPv6 ORPorts in response to EXTEND2 cells containing IPv6 addresses. (Relays always prefer existing canonical connections, which may be over IPv4.)
Make these relays declare the protover from legacy/trac#24404 (moved).
Make them implement the IPv4/IPv6 Extend behviour from legacy/trac#24404 (moved).