When converting postgres boolean data type it was defaulting to the
parquet parquet.string() data type. Pyarrow would then raise an error
when attempting to convert the boolean data type into its string.

Changing the PostgresToGCSOperator class map `data_type` to convert
postgres boolean type to bigquery `BOOL` data type which then maps to
the parquet `pa.bool_()` data type when `_convert_parquet_schema` is
called.