Add ERA files consumer

[dapplion]

For nodes that checkpoint sync, provide an optional flag --era-files-dir which a syncing archive node will use to read blocks from and skip backfilling from the network.

An alternative implementation is to use ERA files on a genesis sync, but this mode is discouraged. Using the backfill strategy the node can verify the ERA files independently, and is simpler to implement.

TODOs

• Support blobs / data columns

Testing

TODO

[dapplion]

Closing in favor of #62 (startup bulk import approach)