fix: read execution_strategy from event directly

[Sekhmet]

Summary

It's stored directly in the event, not under proposal.

Closes: #503

How to test

1. Ideally test with fix: restore preloaded block on error checkpoint-labs/checkpoint#312 (so easily notice if it breaks or not).
2. If you want modify SN_MAIN startBlock to 652679 to speed up testing.
3. Run NETWORK_NODE_URL=https://starknet-mainnet.infura.io/v3/XXX NETWORK=SN_MAIN yarn dev.
4. No more error, indexing doesn't get stuck (if testing using that Checkpoint's PR).