Don't return dictionary encoded booleans

[samuelcolvin]

Required by apache/datafusion#12380 and also apache/datafusion#12511.

But it's probably more efficient to operate on an array of bools than a dictionary anyway.

[adriangb]

I feel like what we need is something like:

use arrow_array::*;
use arrow_cast::cast;
use arrow_schema::DataType;
use arrow::error::ArrowError;

/// Convert a dict array to a non-dict array.
fn unpack_dict_array(array: ArrayRef) -> Result<ArrayRef, ArrowError> {
    match array.data_type() {
        DataType::Dictionary(_, value_type) => {
            cast(array.as_ref(), value_type)
        }
        _ => Ok(array)
    }
}

[samuelcolvin]

superceded (I think) by #43

[samuelcolvin]

We could probably move this logic into json_get since it only applies there.

cc @davidhewitt?

[adriangb]

looks good to me!