feat: support date_series.astype("string[pyarrow]") to cast DATE to STRING (#186)

Author: ajpotts01

bigframes/dtypes.py

@@ -375,7 +375,7 @@ def cast_ibis_value(
  ),
  ibis_dtypes.float64: (ibis_dtypes.string, ibis_dtypes.int64),
  ibis_dtypes.string: (ibis_dtypes.int64, ibis_dtypes.float64),
- ibis_dtypes.date: (),
+ ibis_dtypes.date: (ibis_dtypes.string,),
  ibis_dtypes.Decimal(precision=38, scale=9): (ibis_dtypes.float64,),
  ibis_dtypes.Decimal(precision=76, scale=38): (ibis_dtypes.float64,),
  ibis_dtypes.time: (),

tests/system/small/test_series.py

@@ -2498,6 +2498,7 @@ def test_mask_custom_value(scalars_dfs):
  # with timezone conversions, so we'll allow it.
  ("timestamp_col", pd.ArrowDtype(pa.timestamp("us"))),
  ("datetime_col", pd.ArrowDtype(pa.timestamp("us", tz="UTC"))),
+ ("date_col", "string[pyarrow]"),
  # TODO(bmil): fix Ibis bug: BigQuery backend rounds to nearest int
  # ("float64_col", "Int64"),
  # TODO(bmil): decide whether to fix Ibis bug: BigQuery backend