postgres_fdw: another oddity in costing aggregate pushdown paths

As mentioned in the near thread, I think there is another oversight in
the cost estimation for aggregate pushdown paths in postgres_fdw, IIUC.
 When costing an aggregate pushdown path using local statistics, we
re-use the estimated costs of implementing the underlying scan/join
relation, cached in the relation's PgFdwRelationInfo (ie,
rel_startup_cost and rel_total_cost).  Since these costs wouldn't yet
contain the costs of evaluating the final scan/join target, as tlist
replacement by apply_scanjoin_target_to_paths() is performed afterwards.
 So I think we need to adjust these costs so that the tlist eval costs
are included, but ISTM that estimate_path_cost_size() forgot to do so.
Attached is a patch for fixing this issue.

Best regards,
Etsuro Fujita