Projects should be handled by an IOPushDownRule before applying AggregateProjectMergeRule

It is more efficient to push-down projected fields at an IO level (vs merging with an Aggregate), when supported.

When running queries like:

select SUM(score) as total_score from <TABLE> group by name

Projects get merged with an aggregate, as a result Calc (after an IOSourceRel) projects all fields and BeamIOPushDown rule does know what fields can be dropped, thus not dropping any.