Allow using .map_elements(fn_which_returns_a_2_tuple, return_dtype=pl.Struct(...))

[DeflateAwning]

Request: Allow using .map_elements(fn_which_returns_a_2_tuple, return_dtype=pl.Struct({'field0': pl.String, 'field1': pl.String}))

Description

It would be neat if the following worked!

from typing import Any
import polars as pl

def test_tuple_return_to_struct() -> None:
    df = pl.DataFrame({"id": [1, 2, 3], "value": ["a", "b", "c"]})

    def create_tuple(value: Any) -> tuple[str, str]:
        """Arbitrary function which turns a fixed-length tuple."""
        return f"prefix_{value}", f"suffix_{value}"

    # Attempt to force the fixed-length tuple into a struct.
    df_out = df.with_columns(
        pl.col("value")
        .map_elements(
            create_tuple, return_dtype=pl.Struct({"field0": pl.String, "field1": pl.String})
        )
        .alias("struct_column")
    )

    assert df_out["struct_column"].to_list() == [
        {"field0": "prefix_a", "field1": "suffix_a"},
        {"field0": "prefix_b", "field1": "suffix_b"},
        {"field0": "prefix_c", "field1": "suffix_c"},
    ]

Currently, it errors like:

polars.exceptions.SchemaError: expected output type 'Struct([Field { name: "field0", dtype: String }, Field { name: "field1", dtype: String }])', got 'List(String)'; set `return_dtype` to the proper datatype