Allow sort to accept expressions

[trifox83]

Proposed new feature or change

I'm trying to sort a column by custom sort order via expression.

$expression = "FIELD(country_code, '" . $fields . "')");

return Sort::only([
    'country_code' => [
        'asc'  => [$expression => SORT_ASC],
        'desc' => [$expression => SORT_DESC],
    ],
]);

But \Yiisoft\Data\Reader\Sort::only() does not accept expression.