[tor-bugs] #33816 [Core Tor/Tor]: Fill in missing IPv6 addresses in extend cells

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#33816: Fill in missing IPv6 addresses in extend cells
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     Reporter:  teor           |      Owner:  (none)
         Type:  task           |     Status:  new
     Priority:  Medium         |  Milestone:  Tor: 0.4.4.x-final
    Component:  Core Tor/Tor   |    Version:
     Severity:  Normal         |   Keywords:  ipv6, prop311, outreachy-ipv6
Actual Points:                 |  Parent ID:  #33220
       Points:  1              |   Reviewer:
      Sponsor:  Sponsor55-can  |
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 When an extend cell is missing an IPv6 address, tor relays could use the
 IPv6 address from the consensus. (If there is one.)

 Similarly, if the client only supplied an IPv6 address, the relay could
 add an IPv4 address.

 Relays already add ed25519 keys during an extend, when the client only
 supplies the RSA fingerprint.

 This change helps obfuscate:
 * whether clients know the IPv6 addresses of relays,
 * which clients implement sending IPv6 addresses in extends, and
 * which clients are configured to send IPv6 addresses in extends.

 It has a minor impact on testing:
 * increases the number of IPv6 extends, but
 * decreases the number of IPv4-only extends.

 This change can be made in circuit_extend():
 https://github.com/torproject/tor/pull/1801/files#diff-
 84b529c5e46d955c02d683463cd3317bR230

 By calling a function that works like circuit_extend_add_ed25519_helper(),
 but adds IP addresses instead:
 https://github.com/torproject/tor/pull/1801/files#diff-
 84b529c5e46d955c02d683463cd3317bR77

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33816>
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