Keep execution payload during historical backfill when prune-payloads set to false

[chong-he]

Issue Addressed

• Store execution payloads during backfill if --prune-payloads false #6510

Proposed Changes

• Keep execution payload during historical backfill when --prune-payloads false is set
• Add a field in the historical backfill debug log to indicate if execution payload is kept
• Add a test to check historical blocks has execution payload when `--prune-payloads false is set
• Very minor typo correction that I notice when working on this

Additional Info

• I have done manual testing that when the flag is set to false, we see that the number of keys returned:
  lighthouse db inspect --column exp --output sizes --datadir path is around the range of: head_slot - oldest_block_slot, so I think it works

Thank you @michaelsproul for the hints in testing