Add ARRAY_UNION_SUM analogous to MAP_UNION_SUM

[kaikalur]

WE have map_union_sum but not array_union_sum. I see some users going through hoops to do that. So I feel it'll be good to add that - just elementwise sum of arrays

select array_union_sum(x) from values( array[1,2,3], array[10,5,4,1],array[9,null,5,4])

should give:

array[20, 7,12,5]