Support for passing Authorization header for secured Airflow

[szczeles]

This change provides ability to set Authorization header while accessing Airflow API. The token can be injected via the environment variable, in the mlflow-like way here: https://github.com/mlflow/mlflow/blob/master/mlflow/utils/rest_utils.py#L44 and here: https://github.com/mlflow/mlflow/blob/master/mlflow/tracking/_tracking_service/utils.py#L125

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Keep in mind:

• [x] Documentation updates
• [x] Changelog updates

[szczeles]

@michalzelechowski-getindata Great idea, I have added a documentation with sample Istio filter (as authorization layer) and plugin usage. Please have a look, if everything looks clear I will merge PR and release new version

[michalzelechowski-getindata]

@szczeles , that's just awesome!