Support aggregation result as order field

[windslinux]

What would you like to be added:
Sometimes we need aggregration result as order field, which is supported by sql, but not supported by graphjin.

Why is this needed:
An example as follows, it's not supported yet.
orderTest := `
query getPurchases {
purchases(
order_by: {
sum_quantity: asc
}) {
sum_quantity
customer_id
}
}