This Rule prunes columns and map keys and set to loader. This rule depends
on MapKeysPruneHelper to calculate what keys are required for a loader,
and ColumnPruneHelper to calculate the required columns for a loader. Then
it combines the map keys and columns info to set into the loader.
Get the transformer for this rule. Abstract because the rule
may want to choose how to instantiate the transformer.
This should never return a cached transformer, it should
always return a fresh one with no state.