Currently `SlackWebhookHook` gets the token from `extra` but it would be more secure and intuitive to have it in password.

https://github.com/apache/airflow/blob/4fb5c017fe5ca41ed95547a857c9c39efc4f1476/airflow/providers/slack/hooks/slack_webhook.py
airflow/providers/slack/hooks/slack_webhook.py
"""
        Given either a manually set token or a conn_id, return the webhook_token to use.
        :param token: The manually provided token
        :type token: str
        :param http_conn_id: The conn_id provided
        :type http_conn_id: str
        :return: webhook_token to use
        :rtype: str
        """
        if token:
            return token
        elif http_conn_id:
            conn = self.get_connection(http_conn_id)
            extra = conn.extra_dejson
            return extra.get('webhook_token', '')
        else:
            raise AirflowException('Cannot get token: No valid Slack webhook token nor conn_id supplied')
    def _build_slack_message(self) -> str: