[FEA] Support GPU-accelerated IO writers in Polars

[beckernick]

Currently, Polars can use GPU-accelerated readers but not writers.

Unlike Lazy API queries that materialize a result in memory, the sink_* APIs don't require calling collect() (e.g., sink_parquet) to trigger execution and are explicitly tied to Polars streaming mode, which is undergoing a significant rewrite.

Given that, we may want to hold off for now. Filing for tracking and discussion purposes.