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 #24404 (moved).

Make them implement the IPv4/IPv6 Extend behviour from #24404 (moved).

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