feat: add json_arrow_type parameter to allow overriding the JSON data type in to_arrow and to_arrow_iterable

[tswast]

Follow-up to #1876 (comment)

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Fixes #<issue_number_goes_here> 🦕

[tswast]

Currently fails with

        rows = google.cloud.bigquery.table.RowIterator(
            client=None,
            api_request=None,
            path=None,
            schema=[
                bigquery.SchemaField.from_api_repr(field)
                for field in resource["schema"]["fields"]
            ],
            first_page_response=resource,
        )
>       records = rows.to_arrow(json_arrow_type=db_dtypes.JSONArrowType())

tests/unit/test_table_arrow.py:275: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
google/cloud/bigquery/table.py:2227: in to_arrow
    return pyarrow.Table.from_batches(record_batches, schema=arrow_schema)
pyarrow/table.pxi:4994: in pyarrow.lib.Table.from_batches
    ???
pyarrow/error.pxi:155: in pyarrow.lib.pyarrow_internal_check_status
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   pyarrow.lib.ArrowInvalid: Schema at index 0 was different: 
E   json: extension<dbjson<JSONArrowType>>
E   reference: struct<uri: string, authorizer: string, version: string, details: extension<dbjson<JSONArrowType>>>
E   repeated_json: list<item: extension<dbjson<JSONArrowType>>> not null
E   vs
E   json: string
E   reference: struct<uri: string, authorizer: string, version: string, details: string>
E   repeated_json: list<item: string> not null

pyarrow/error.pxi:92: ArrowInvalid

I think we need to call https://arrow.apache.org/docs/python/generated/pyarrow.compute.cast.html instead of relying on the Table.from_batches constructor.

[tswast]

I think we'll need some kind of two-phase thing where we take the schema as we would get from the BQ Storage Read API and manually cast each field/subfield into the desired type to make it work.

I don't think the approach I'm using in this PR as of now will work. It'll have to do some post-processing on the arrow Table we'd normally get.

[tswast]

Closing as obsolete. Ideally we stay consistent with the BQ Storage API. We should revisit this if the BQ Storage API adds support for the JSON extension type.