Support also configuring BigQuery in JSON

[jonespm]

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Describe your problem or feature you'd like added

For the BigQuery config in the cron we rely on the bigquery defined in a JSON file referenced in the environment.

GOOGLE_APPLICATION_CREDENTIALS=/secrets/bq_cred.json

However in the latest versions of the library, we can also pull the settings from the main config instead:

self.bigquery_client = bigquery.Client.from_service_account_json(settings.BIGQUERY_SERVICE_ACCOUNT_JSON)

This might simply the setup a little bit. The old method should still work.