Mask sensitive proxy values in airflow config

[wangyimosquito]

Description of changes

• Fix CVE identified to mask sensitive "proxy" and "proxies" info in connection by adding them to Airflow core config as default. Also updated the related unit tests.
• Users can also choose to override this config accordingly.

Testing

• Deployed 2.11.0 image and verified by running a DAG that (1) checks the core config is set; (2)shows masked proxy info.

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