[Feature] Built-in execution context variables

[imkebe]

I want to create the script that will be executed using handlerOn. It' would be great to have built-in execution context variables for each DAG like:

• DAG_SCHEDULER_LOG_PATH
• DAG_EXECUTION_LOG_PATH
• DAG_REQUEST_ID

to be able to pass them as command args.

handlerOn:
  failure:
    command: "echo There was an error while doing ${DAG_REQUEST_ID} request"

[yohamta]

Thanks for the great suggestion. I think this feature would be really valuable. Let's implement this. To avoid naming conflict, we could use a prefix DAGU_* or something similar.