[FEATURE] Notification action validation

[Angie-Zhang]

Clarify Notification action validation conditions and implement it.

Meta Feature
#587

We have 2 levels of notification: policy level and action level.

Policy Level:

"policy": {
     ......
     "error_notification": {
         "destination": {
             "slack": {
                 "url": "https://hooks.slack.com/services/xxx/xxxxxx"
             }
         },
         "message_template": {
             "source": "the index is {{ctx.index}}",
             "lang": "mustache"
         }
    }
  ......
}

Action Level:

{
    ......
        "states": [
            {
                "name": "dead_notification",
                "actions": [
                    {
                        "retry": {
                            "count": 3,
                            "backoff": "exponential",
                            "delay": "1m"
                        },
                        "notification": {
                            "channel": {
                                "id": "NYXbx4UBlNI_fIOWo8Xm"
                            },
                            "message_template": {
                                "source": "the index {{ctx.index}} has been deleted.",
                                "lang": "mustache"
                            }
                        }
                    }
                ],
                "transitions": []
            }
        ],
        ......
    }
}

Error Prevention only validates notification at action level.

Validation Conditions
ISM sending notifications depends on Notification plugin. For each request to notification plugin, it checks the response.
The Notification action errors can only be prevent when configuring notification channels.
Thus ISM Error Prevention does not validate notification action.