[State sync engine] Add new "ExecutionDataCID" field to ExecutionResult

[synzhu]

Add a ExecutionDataCID field to execution result, so that consensus followers can know how to request the state diff for a given block over bitswap.

Each time an execution node computes a new ExecutionData for a block, it can forward it to the ExecutionDataStorer to be stored into the blockstore, and include the returned CID in the execution result.

Note: This may introduce additional latency / CPU usage on execution nodes, we should benchmark to see how much the performance impact will be.

[huitseeker]

Note that the verifiability of the CID in question will eventually need to be checked by verification nodes, but we can do something faster for the moment, which is to check the data against the authoritative hashes contained in the ExecutionReceipt post download. More details here:
https://www.notion.so/dapperlabs/State-sync-engine-bed32c0caba94913b21925f6bc090aaf#d4ef9d5452424412b69f149e40ad35d1

[synzhu]

Hey team! Please add your planning poker estimate with ZenHub @franklywatson @m4ksio @peterargue @vishalchangrani