SSL issue with SlackWebhookOperator on K8S

[benbenbang]

Recently our team is testing all the days to check everything will work fine on airflow on k8s. However, one of the issues we didn't have before is that the SSL issue happens in the POD. The only one for us to fix it is turn on verify=True for all the HTTP related hook. But except SlackWebhookOperator, it was hardcoded with proxy and cannot take any extra option anymore.

https://airflow.apache.org/docs/stable/_modules/airflow/contrib/hooks/slack_webhook_hook.html

If there's no PR for this, I would like to open a PR to do a quick enhancement on this operator. Advice and any other comments are all welcome. Thanks in advance.

[boring-cyborg]

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