feat(resharding): flat storage resharding deltas (#12271)

Implementation of flat storage deltas split for flat storage resharding.

Before this change, only flat storage key pairs at flat head height were
split during resharding. Now flat storage deltas are part of the
splitting process as well.
On a high level, it works by concatenating the iterator over flat
storage and the iterator over deltas for all block heights between chain
head an flat storage head.

Part of #12174