feat(relayer): add indexer for MAX() in MySQL

[xiaodino]

It's to optimize the query

SELECT `id`, `block_height`, `hash`, `event_name`, `chain_id`
FROM `processed_blocks`
WHERE `block_height` = (
  SELECT MAX(`block_height`)
  FROM `processed_blocks`
  WHERE `chain_id` = ? AND `event_name` = ?
)

The subquery filters rows based on chain_id and event_name and then finds the maximum block_height. An index that includes these three columns can significantly improve the performance of the subquery by allowing MySQL to quickly locate the rows that match the given chain_id and event_name, and then efficiently determine the maximum block_height among those rows.