Add `to_ducksql` to the Python api

> We _do_ have implementation-specific SQL converters in the Rust crate (e.g. [`to_ducksql`](https://github.com/developmentseed/cql2-rs/blob/8401fbfebdca7c7b64720252f812149d239c0b2d/src/duckdb.rs#L8)), though that particular method doesn't exist in the Python side yet (something we should rectify @bitner).

_Originally posted by @gadomski in [#235](https://github.com/developmentseed/cql2-rs/issues/235#issuecomment-4718565277)_