Use LITERAL_AGG to simplify SubQueryRemoveRule by avoiding the extra Project

For example, when rewriting sub-queries (see SubQueryRemoveRule) we introduce add "true as indicator" to the SELECT clause of sub-queries. It can be used to detect rows generated by an outer join. If it is an aggregate query, we would have to write "min(true) as indicator", which necessitates an extra Project below the Aggregate to provide the "true" value. The LITERAL_AGG aggregate function allows us to avoid the extra Project.