dag: Add support for Slack notification in DAG definitions

[yottahmd]

This feature introduces a new slackOn configuration field that allows users to enable Slack alerts for DAG execution events such as success and failure. Users can define webhook URLs for each event.

Example:

slackOn:
  success: true
  failure: true
slack:
  success:
    webhookUrl: https://hooks.slack.com/services/T00000000/B00000000
  failure:
    webhookUrl: https://hooks.slack.com/services/T00000000/B00000000