Add splats to polars_util.rs

[jleibs]

#736 added support for splats to re_query

We should go back and add comparable logic to polars_util for completeness, but the ergonomics of doing so is surprisingly painful. I think the answer is going to be to pull in polar-lazy so we can use https://pola-rs.github.io/polars/polars_lazy/dsl/enum.Expr.html#method.repeat_by

Trying to do the equivalent directly on top of raw series and dataframes was not working well.

[teh-cmc]

We're moving away from polars.