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
tests/providers/slack/hooks/test_slack_webhook.py
_config = {
        'http_conn_id': 'slack-webhook-default',
        'webhook_token': 'manual_token',
        'message': 'Awesome message to put on Slack',
        'attachments': [{'fallback': 'Required plain-text summary'}],
        'blocks': [{'type': 'section', 'text': {'type': 'mrkdwn', 'text': '*bold text*'}}],
        'channel': '#general',
        'username': 'SlackMcSlackFace',
        'icon_emoji': ':hankey:',
        'icon_url': 'https://airflow.apache.org/_images/pin_large.png',
        'link_names': True,
        'proxy': 'https://my-horrible-proxy.proxyist.com:8080',
    }
    expected_message_dict = {
        'channel': _config['channel'],
        'username': _config['username'],
        'icon_emoji': _config['icon_emoji'],
        'icon_url': _config['icon_url'],
        'link_names': 1,
        'attachments': _config['attachments'],
        'blocks': _config['blocks'],
        'text': _config['message'],
    }
    expected_message = json.dumps(expected_message_dict)
    expected_url = 'https://hooks.slack.com/services/T000/B000/XXX'
    expected_method = 'POST'
    def setUp(self):