Single block iterator for ExEx backfill

[shekhirin]

Describe the feature

Currently, we only have an iterator that iterates in batches and yields the next iterator value when the batch has finished executing:

reth/crates/exex/exex/src/backfill.rs

Lines 68 to 69 in a3ab3d0

	/// It implements [`Iterator`] that executes blocks in batches according to the provided thresholds
	/// and yields [`Chain`]

We need to add another iterator that yields just the (Block, ExecutionOutcome). I propose the following API:

let factory = BackfillJobFactory::new(...);
let backfill = factory.backfill(range).into_single_blocks();
for (block, outcome) in backfill {
   ...
}

into_single_blocks will return another type of iterator that executes blocks one by one, and returns (Block, ExecutionOutcome) instead of the whole Chain.

Additional context

No response

[greged93]

hey @shekhirin, I'm down to take this if I may