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**Apache Airflow version**: 1.10.12


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**: Google Cloud Composer 1.13.3

- **Cloud provider or hardware configuration**: Google Cloud Composer

**What happened**: All Google Marketing Platform Display and Video 360 SDF operations fail (other than creating the SDF). The "GoogleDisplayVideo360GetSDFDownloadOperationSensor" fails every time it runs in any DAG I have tested with the error 'Resource' object has no attribute 'operation'. 

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**What you expected to happen**: Given a valid SDF operation object that has been created, the operator should check if that object is ready to download. Then, the GoogleDisplayVideo360SDFtoGCSOperator should download the operation. However, the underlying hook that is being used is trying to reference an "operation" instead of "operations," per the SDF API specifications (see [here](https://developers.google.com/display-video/api/reference/rest/v1/sdfdownloadtasks.operations/get)). Thus, the requests always fail (as there is no "operation" attribute of the API).

<!-- What do you think went wrong? -->

**How to reproduce it**:
<!---

To reproduce, try to create a valid DAG that creates a new SDF file and then downloads it. I have tested with my own DAGs which are based on the example DAG provided in that code. I believe that if you run the SDF portion of the [Display and Video example DAG](https://github.com/apache/airflow/blob/master/airflow/providers/google/marketing_platform/example_dags/example_display_video.py) in the docs then the issue will occur, regardless of environment.

## Install minikube/kind

- Minikube https://minikube.sigs.k8s.io/docs/start/
- Kind https://kind.sigs.k8s.io/docs/user/quick-start/

If this is a UI bug, please provide a screenshot of the bug or a link to a youtube video of the bug in action

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--->


**Anything else we need to know**: The patch is very simple -- I have already tested by patching my code locally and confirming that the patch fixes the problem. I will submit a pull request shortly.

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airflow/providers/google/marketing_platform/hooks/display_video.py
"""Retrieves connection to DisplayVideo."""
        if not self._conn:
            http_authorized = self._authorize()
            self._conn = build(
                "doubleclickbidmanager",
                self.api_version,
                http=http_authorized,
                cache_discovery=False,
            )
        return self._conn
    def get_conn_to_display_video(self) -> Resource:
        """Retrieves connection to DisplayVideo."""
        if not self._conn:
            http_authorized = self._authorize()
            self._conn = build(
                "displayvideo",
                self.api_version,
                http=http_authorized,
                cache_discovery=False,
            )
        return self._conn
    @staticmethod
    def erf_uri(partner_id, entity_type) -> List[str]: