ESQL: a more accurate node-level reduction estimated row size value

### Description

Following https://github.com/elastic/elasticsearch/pull/106516 merge, [PlannerUtils.dataNodeReductionPlan](https://github.com/elastic/elasticsearch/pull/106516/files#diff-b41d9e84ede469955298c5715cc6f1a0ed4f9a602c59c95761fadfe54ec3a745R91) should come up with a smarter way to determine a better estimation for the row size value.