Support CROSS JOIN UNNEST()

[FallenerFurtherstr]

Description

Hi everyone! I'm new to Polars, and already enjoying sqling lazyframes. I was trying to unnest a column of array into multiple rows of data, just like what CROSS JOIN UNNEST() does in Postgre SQL. So I tried to execute following sql sentence as an example:

WITH sequence AS
(
    SELECT 1 AS id, [0, 1, 2] AS numbers
    UNION ALL SELECT 2 AS id, [4, 6] AS numbers
)
Select id, flattened_numbers
FROM sequences
CROSS JOIN UNNEST(numbers) AS flattened_numbers

This example should yield a result like this:

id	numbers
1	0
1	1
1	2
2	4
2	6

But upon executing this, it throws an error:

polars.exceptions.SQLSyntaxError: Expected array expression, found Identifier(Ident { value: "numbers", quote_style: None, span: Span(Location(9, 23)..Location(9, 30)) })

Seems that Polars doesn't implement this feature (looks like UNNEST always wants an array), or I just do it in a wrong way. Since this operation doesn't affect the overall schema (i.e. it doesn't add new columns, but rows), it would greatly enhance the sql feature if it's possible to do so.

EDIT: Remove redundant header