Added a way to define OrderBy by external parameters

• Added OrderBySpecification
• Added a way to define OrderBy by external parameters

$_GET['sort'] = [['field2', 'ASC'], ['field1', 'DESC'], ['field3' => 'ASC']];

echo $db
->field('t.field1')
->field('t.field2')
->from('t', 'test')
->orderBy(new DBExprOrderBySpec(['field1', 'field2' => 'REVERSE(t.field2)'], $_GET['sort']))
->asString();

SELECT
	t.field1,
	t.field2
FROM
	test t
ORDER BY
	REVERSE(t.field2) ASC,
	field1 DESC