**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
:param bigquery_conn_id: (Deprecated) The connection ID used to connect to Google Cloud.
        This parameter has been deprecated. You should pass the gcp_conn_id parameter instead.
    :type bigquery_conn_id: str
    :param use_legacy_sql: Whether to use legacy SQL (true)
        or standard SQL (false).
    :type use_legacy_sql: bool
    :param location: The geographic location of the job. See details at:
        https://cloud.google.com/bigquery/docs/locations#specifying_your_location
    :type location: str
    :param impersonation_chain: Optional service account to impersonate using short-term
        credentials, or chained list of accounts required to get the access_token
        of the last account in the list, which will be impersonated in the request.
        If set as a string, the account must grant the originating account
        the Service Account Token Creator IAM role.
        If set as a sequence, the identities from the list must grant
        Service Account Token Creator IAM role to the directly preceding identity, with first
        account from the list granting this role to the originating account (templated).
    :type impersonation_chain: Union[str, Sequence[str]]
    :param labels: a dictionary containing labels for the table, passed to BigQuery
    :type labels: dict
    """
    template_fields = (
        'sql',
        'gcp_conn_id',
        'impersonation_chain',
        'labels',
    )
    template_ext = ('.sql',)
    ui_color = BigQueryUIColors.CHECK.value
    def __init__(