fix: Expose FieldsMapper (#23232)

Author: stijnherfst

crates/polars-plan/src/plans/aexpr/mod.rs

@@ -24,6 +24,7 @@ pub use scalar::is_scalar_ae;
 use strum_macros::IntoStaticStr;
 pub use traverse::*;
 mod properties;
+pub use aexpr::function_expr::schema::FieldsMapper;
 pub use builder::AExprBuilder;
 pub use properties::*;