feat: add index config

[blairdrummond]

By default rekor-sidekick follows the tail of the Rekor log. This probably makes sense in most use-cases, but if we're mirroring attestations out to a searchable database, or testing on a self-hosted Rekor instance (which is receiving less traffic) it's helpful to be able to replay the whole Rekor instance or replay it from a certain point.

The new index configuration, when set to -1 (default) just follows the Rekor instance like before, but if you set it to a value >=0, then it replays rekor from that index.

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