allow calling sqlpage functions on all rows returned from a database query

[lovasoa]

Currently sqlpage functions are executed before the query is sent to the database, and their output is sent to the database. To use a value from the database as input in a sqlpage function, one needs to define a variable:

set $x = (select x from y where id = 123);
select sqlpage.some_function($x);

However, the following is not possible:

select sqlpage.some_function(x) from y;

we should lift this limitation.

see https://github.com/lovasoa/SQLpage/discussions/328