**Description**

`BigQueryInsertJobOperator` requires the submission of a `configuration` parameter in the form of a dict.  Unfortunately, if this contains a large SQL query - especially one that is formatted with new lines - then this cannot currently be rendered very nicely in the UI.

<img width="1670" alt="Screenshot 2021-07-15 at 15 39 33" src="https://user-images.githubusercontent.com/967119/125806943-839d57a9-d4a0-492d-b130-06432b095239.png">

**Use case / motivation**

The issue with this is that it's impossible to copy and paste the rendered query out of the Airflow UI, into a BigQuery browser and run it without lots of manual edits which is time wasted when troubleshooting problems.

**Are you willing to submit a PR?**

Yes.  My current thought process around this would be to add an optional SQL parameter to the operator which, if provided, would be added into the configuration and could therefore have its own template field and SQL renderer.

e.g.

<img width="1570" alt="Screenshot 2021-07-14 at 14 18 09" src="https://user-images.githubusercontent.com/967119/125808200-5f30b8f4-4def-48a7-8223-82afdc65c973.png">


airflow/providers/google/cloud/operators/bigquery.py
"""
    Checks that the values of metrics given as SQL expressions are within
    a certain tolerance of the ones from days_back before.
    This method constructs a query like so ::
        SELECT {metrics_threshold_dict_key} FROM {table}
        WHERE {date_filter_column}=<date>
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:BigQueryIntervalCheckOperator`
    :param table: the table name
    :type table: str
    :param days_back: number of days between ds and the ds we want to check
        against. Defaults to 7 days
    :type days_back: int
    :param metrics_thresholds: a dictionary of ratios indexed by metrics, for
        example 'COUNT(*)': 1.5 would require a 50 percent or less difference
        between the current day, and the prior days_back.
    :type metrics_thresholds: dict
    :param use_legacy_sql: Whether to use legacy SQL (true)
        or standard SQL (false).
    :type use_legacy_sql: bool