Update padding negotiation with preference menu, response delays
As we add more padding machines across different Tor versions, padding negotiation will become more and more likely to fail. We could send a preference list to the relay instead, so that if any of the machines we could use are supported, they would be chosen in preference order.
We could also add a delay field asking the relay to delay before responding, as an alternate way to deal with concerns in #30172 (moved).
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