fix(substrait): Allow scalar subquery in projection

[bvolpato-dd]

Which issue does this PR close?

• Closes [substrait] Scalar subquery in select not supported #18066.

Rationale for this change

Recent validations added at #15334 started to cause problems for some LEFT JOIN queries that don't have conditions, for example:

SELECT a, (SELECT MAX(b) FROM data2) as max_b FROM data

This query gets optimized as a left join without conditions.

This basically follows the same approach from #15334 :

datafusion/datafusion/optimizer/src/scalar_subquery_to_join.rs

Line 345 in a65a2cb

vec![Expr::Literal(ScalarValue::Boolean(Some(true)), None)],

What changes are included in this PR?

For joins without conditions, use cross_join if inner, otherwise use a filter with Boolean(true)

Are these changes tested?

Added a roundtrip test case.

Are there any user-facing changes?

n/a

[bvolpato-dd]

Sorry for the mess, opened with a different account by mistake.