Expression `column_refs` does not return outer referenced columns in subqueries

### Describe the bug

Calling `column_refs()` on a subquery expression returns no column references, even if the subquery contains outer references.

PushDownFilter optimization uses the `column_refs()` to compare against the columns returned from `prevent_predicate_push_down_columns()` on an extension node. The effect of this is that a subquery in a filter may get pushed below an extension node when it shouldn't.

### To Reproduce

```rust
let outer_ref = Expr::OuterReferenceColumn(DataType::Int64, Column::new(None::, "a"));
let plan = LogicalPlanBuilder::empty(true)
.project(vec![outer_ref.clone()])
.unwrap()
.build()
.unwrap();

let subq = Subquery {
subquery: Arc::new(plan),
outer_ref_columns: vec![outer_ref],
};

let expr = Expr::Exists(Exists::new(subq, false));

println!("{:?}", expr.column_refs());
```

### Expected behavior

`column_refs()` should return the outer columns used by the subquery expression.

### Additional context

The relevant code is [here](https://github.com/apache/datafusion/blob/main/datafusion/expr/src/expr.rs#L1533). It just collects `Expr::Column`s within self.

Start in datafusion/expr/src/expr.rs around line 1533 and reproduce the issue with the example expression containing an outer reference inside a subquery. Trace how column_refs() collects references, then verify that it returns the subquery's outer columns and that the regression is covered by relevant expression tests.

Written by the indexing model from the issue text.