feat: Generic task and DAG callbacks

[bagemt]

Defining what to do when task or DAG finishes is disjointed.

This is proposal to add generic callbacks:, you can have multiple of them depending on and per the status success / failure / skipped

mailOn, #758, #688 can be reclassified as callbacks

Additional callbacks can include generic webhooks and execution of user defined callbacks from command:

[ghansham]

I think we can use onSuccess, onFailure, onCancelled hooks to plugin the callbacks/scripts like messaging users via social apps, mail log files and what not.

[bagemt]

That's the idea, with the user defined callback you can execute from scripts, whatever you like, depending on the status

• onSuccess
• onFailure
• onCancelled
• onSkipped

something like

- name: step1
  callback:
    onFailure:
      - type: script
        command: python upload_logs.py
      - type: mail
        message:
        recipients:
        ...
    onSuccess:
      - type: webhook
        url:
      - type: slack (slack webhook)
    ....

To be considered is what variables are accessible to the callbacks

• all from the DAG and TASKS
• DAG ID / Name, Task name, everything from Run Status

[ghansham]

One possible list of tasks could be we may have access to something like failed steps list, their stdout/stderr logs, return codes, etc easily accessible via variables and tasks defined to tar/zip and share. In sharing step, overall dag run status (start/end time) with success or failure details (failed step details) on every dag run or may be dag run reports on daily or monthly basis.