Google secret backend impersonalization feature

[makiwino]

This feature enables to use impersonation_chain in kwargs in env variables setting Google secret manager backend.
Example of the use of new feature:

AIRFLOW__SECRETS__BACKEND=airflow.providers.google.cloud.secrets.secret_manager.CloudSecretManagerBackend
AIRFLOW__SECRETS__BACKEND_KWARGS='{"impersonation_chain":"service_account@project_id.iam.gserviceaccount.com","project_id":"project_id","connections_prefix": "airflow-connections", "variables_prefix": "airflow-variables"}'

Why this change is needed?
Our Airflow deployment service account and team developers accounts share one main team serviceaccount via serviceAccountTokenCreator permission. This team serviceaccount has accesses to particular services like BQ, GCS, Google secrets. While BQ and GCS operators has impersonalisation_chain arg, google secret manager backend doesnt support this feautre. Using this new feature all approaches are aligned and it is possible to follow this good pattern.

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[potiuk]

We need unit tests for that I thinnk

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.