BigQuerySource external data source support in non-US regions

I am attempting to query an [external data source](https://cloud.google.com/bigquery/external-data-sources), a MySQL database that is exposed via the BigQuery API, located in the EU region. I have the following format query string:

```

query = """
SELECT *
FROM EXTERNAL_QUERY("my-project-one-253518.eu.external-source",

"SELECT * FROM my schema.mytable;");
"""

```

And the following pipeline instantiation:

```

pcoll = p | "Load " + name >> beam.io.Read(
beam.io.BigQuerySource(query=query, use_standard_sql=True)

)

```

When run this, I see the following output:
```

WARNING:root:Dataset my-project-two:temp_dataset_f07dd1398b0443edaa67c360f5be6958 does not exist so
we will create it as temporary with location=None
ERROR:root:Exception at bundle , due to an exception.
Traceback (most recent call last):
File "venv/lib/python3.7/site-packages/apache_beam/runners/direct/executor.py",
line 345, in call
finish_state)
File "venv/lib/python3.7/site-packages/apache_beam/runners/direct/executor.py",
line 385, in attempt_call
result = evaluator.finish_bundle()
File "venv/lib/python3.7/site-packages/apache_beam/runners/direct/transform_evaluator.py",
line 323, in finish_bundle
bundles = _read_values_to_bundles(reader)
File "venv/lib/python3.7/site-packages/apache_beam/runners/direct/transform_evaluator.py",
line 310, in _read_values_to_bundles
read_result = [GlobalWindows.windowed_value(e) for e in reader]

File "venv/lib/python3.7/site-packages/apache_beam/runners/direct/transform_evaluator.py", line 310,
in
read_result = [GlobalWindows.windowed_value(e) for e in reader]
File "venv/lib/python3.7/site-packages/apache_beam/io/gcp/bigquery_tools.py",
line 937, in __iter__
flatten_results=self.flatten_results):
File "venv/lib/python3.7/site-packages/apache_beam/io/gcp/bigquery_tools.py",
line 710, in run_query
page_token, location=location)
File "venv/lib/python3.7/site-packages/apache_beam/utils/retry.py",
line 209, in wrapper
return fun(*args, **kwargs)
File "venv/lib/python3.7/site-packages/apache_beam/io/gcp/bigquery_tools.py",
line 384, in _get_query_results
response = self.client.jobs.GetQueryResults(request)
File "venv/lib/python3.7/site-packages/apache_beam/io/gcp/internal/clients/bigquery/bigquery_v2_client.py",
line 312, in GetQueryResults
config, request, global_params=global_params)
File "venv/lib/python3.7/site-packages/apitools/base/py/base_api.py",
line 731, in _RunMethod
return self.ProcessHttpResponse(method_config, http_response, request)

File "venv/lib/python3.7/site-packages/apitools/base/py/base_api.py", line 737, in ProcessHttpResponse

self.__ProcessHttpResponse(method_config, http_response, request))
File "venv/lib/python3.7/site-packages/apitools/base/py/base_api.py",
line 604, in __ProcessHttpResponse
http_response, method_config=method_config, request=request)
apitools.base.py.exceptions.HttpBadRequestError:
HttpError accessing :
response: <{'vary': 'Origin, X-Origin, Referer', 'content-type': 'application/json; charset=UTF-8',
'date': 'Tue, 02 Jun 2020 11:29:27 GMT', 'server': 'ESF', 'cache-control': 'private', 'x-xss-protection':
'0', 'x-frame-options': 'SAMEORIGIN', 'x-content-type-options': 'nosniff', 'alt-svc': 'h3-27=":443";
ma=2592000,h3-25=":443"; ma=2592000,h3-T050=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q049=":443";
ma=2592000,h3-Q048=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443";
ma=2592000; v="46,43"', 'transfer-encoding': 'chunked', 'status': '400', 'content-length': '354', '-content-encoding':
'gzip'}>, content <{
"error": {
"code": 400,
"message": "Cannot read and write in different
locations: source: EU, destination: US",
"errors": [
{
"message": "Cannot read
and write in different locations: source: EU, destination: US",
"domain": "global",

"reason": "invalid"
}
],
"status": "INVALID_ARGUMENT"
}
}

```

Which likely indicates to me that the logic Beam uses to impute the zone in which to create the temporary dataset fails when confronted with the special syntax for external queries. Therefore it seems like the zone should be exposed as a parameter of BigQuerySource.

Imported from Jira [BEAM-10172](https://issues.apache.org/jira/browse/BEAM-10172). Original Jira may contain additional context.
Reported by: muscovite bob.

apache_beam/io/gcp/bigquery_tools.py から開始し、traceback に示されているクエリ結果のパスを通って BigQuerySource を追跡します。EU のソースを使って EXTERNAL_QUERY の例を再現し、一時データセットのロケーションをデフォルトで US にするのではなく、ソースリージョンと一致させられることを確認します。

索引モデルが issue の本文から書いたものです。