[State sync engine] ExecutionData requester engine

[synzhu]

[State sync engine] ExecutionData requester engine

[franklywatson]

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[synzhu]

@peterargue After our discussion today, I realize that this engine actually doesn't need to communicate directly with the network.

The initial version just needs to subscribe to the Finalization Distributor and go through all the seals in each finalized block to request all the sealed Execution Data CIDs.

In the optimized version, we can modify the Ingestion engine to forward execution receipts to this new engine via SubmitLocal, which can optimistically trigger download of the Execution Data. We continue to subscribe to block finalizations, and clear the datastore TTL for an Execution Data only once it is sealed (allowing the rest to expire on their own).