Adds a subcommand: `geth snapshot export-preimages`, to export preimages of every hash found during a snapshot enumeration: that is, it exports _only the active state_, and not _all_ preimages that have been used but are no longer part of the state.
This tool is needed for the verkle transition, in order to distribute the preimages needed for the conversion. Since only the 'active' preimages are exported, the output is shrunk from ~70GB to ~4GB.
The order of the output is the order used by the snapshot enumeration, which avoids database thrashing. However, it also means that storage-slot preimages are not deduplicated.
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