select multiple columns in a single `Expr`

### Is your feature request related to a problem or challenge?

I want to create a udf that can select multiple functions at once, such as a `COLUMNS()` function.

`select COLUMNS('number\d+') from my_table`.

Currently this is not possible due to the fact that udfs can only ever output a single `Expr`.

### Describe the solution you'd like

Since it would be quite a massive overhaul to refactor all of the planning and udf logic to return `Vec`, I propose adding a new variant to `Expr`.

`Expr::Columns(Vec)`.

This seems like the least invasive way to support selecting multiple columns in a single expr.

### Describe alternatives you've considered

I'm open for alternatives, but I am not aware of any.

### Additional context

Polars has this variant in their `Expr`

https://github.com/pola-rs/polars/blob/9fec2ecb6d4295969e1d155b386ee82db08745a1/crates/polars-plan/src/dsl/expr.rs#L72

Start by tracing the Expr, UDF, and planning logic referenced in the issue, and compare the proposed Expr::Columns(Vec) with the linked Polars Expr variant. Define how one expression can select multiple columns without refactoring every planner and UDF return path; done means the requested COLUMNS() query can be represented and planned.

Written by the indexing model from the issue text.