O1.1 - Propose and implement IPv6 ORPort reachability checks on relays

See Proposal 311: Tor Relay IPv6 Reachability: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt

The child tickets are in proposal section order, but they will probably be implemented in this order:

Pre-Implementation Improvements:

Require IPv6 tests in Travis CI (O1.3, #33195 (moved))
Test IPv4 Reachability using Chutney (O1.3, #33232 (moved))

Implementation:

Allow Relay IPv6 Extends
Declare Support for Subprotocol Version "Relay=3"
Check Relay and Bridge IPv6 ORPort Reachability

Internal Testing:

Test IPv6 ORPort Reachability using Chutney (O1.3, #33228 (moved))
Test Legacy Relays Accept IPv6 Extends using Chutney (O1.3, #33231 (moved))

Public Tor Network Testing:

Test IPv6 ORPort Reachability on the Tor Network
Ask Relay Operators to Test IPv6 Reachability

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