Helper to get all of the proof CIDs from a UCAN chain

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Now that all proofs are referenced by their CID, and with consideration of the UCAN as Bearer Token spec's affordance for transmitting UCANs wholesale via the ucan header, it would be very helpful if we had a mechanism that enabled users to easily collect proof CIDs from a UCAN chain. It may be nice to design it such that proofs that are already thought to be well-known by all parties may be skipped as they are encountered.