Add a probability-to-apply field for circuitpadidng machines
In #28634 (moved), we realized that we may want to make some fraction of pre-built GENERAL and HS_VANGUARDS circuits look like padded onion service circuits, as a defense in depth against a classifier that can still recognize our specially padded onion service circuits as, well, special, and still interesting.
But we don't want to make all general circuits look this way. Just some fraction. So it would be nice if the machine conditions could somehow toss a coin to decide to apply the machine to a circuit. Unfortunately, right now the conditions are memoryless, so we have nothing that can say "you already tossed the coin", but we could special case just this to have a flag on the circuit or something.
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