[Enh]: Define DType Key for Categorical data types

[ELC]

Please describe the purpose of the new feature or describe the problem to solve.

See this snippet

def reverse_translate_dtype(dtype: dtypes.DType | type[dtypes.DType]) -> Any:
    from narwhals import dtypes

    pa = get_pyarrow()

    if isinstance_or_issubclass(dtype, dtypes.Float64):
        return pa.float64()
    if isinstance_or_issubclass(dtype, dtypes.Float32):
        return pa.float32()
    if isinstance_or_issubclass(dtype, dtypes.Int64):
        return pa.int64()
    if isinstance_or_issubclass(dtype, dtypes.Int32):
        return pa.int32()
    if isinstance_or_issubclass(dtype, dtypes.Int16):
        return pa.int16()
    if isinstance_or_issubclass(dtype, dtypes.Int8):
        return pa.int8()
    if isinstance_or_issubclass(dtype, dtypes.UInt64):
        return pa.uint64()
    if isinstance_or_issubclass(dtype, dtypes.UInt32):
        return pa.uint32()
    if isinstance_or_issubclass(dtype, dtypes.UInt16):
        return pa.uint16()
    if isinstance_or_issubclass(dtype, dtypes.UInt8):
        return pa.uint8()
    if isinstance_or_issubclass(dtype, dtypes.String):
        return pa.string()
    if isinstance_or_issubclass(dtype, dtypes.Boolean):
        return pa.bool_()
    if isinstance_or_issubclass(dtype, dtypes.Categorical):
        # TODO(Unassigned): what should the key be? let's keep it consistent
        # with Polars for now
        return pa.dictionary(pa.uint32(), pa.string())
    if isinstance_or_issubclass(dtype, dtypes.Datetime):
        # Use Polars' default
        return pa.timestamp("us")
    if isinstance_or_issubclass(dtype, dtypes.Duration):
        # Use Polars' default
        return pa.duration("us")
    if isinstance_or_issubclass(dtype, dtypes.Date):
        return pa.date32()
    msg = f"Unknown dtype: {dtype}"  # pragma: no cover
    raise AssertionError(msg)

[FBruzzesi]

I know this is tagged as TODO, but to me it seems pretty ok as it is 👀