Aggregation pushdown with coercions

[findepi]

Currently some connectors, e.g. PostgreSQL, can do aggregation pushdown for sum(a_bigint).
However, they cannot do sum(an_integer), because it is actually sum(CAST(an_integer AS bigint)).

We could define a sum(integer) function and let the connector accept it in aggregation pushdown.

Ideally, however, we should have ability to pushdown expressions in the context of coercions.