executor: Add DAG_DETAILS_JSON for access to DAG metadata within tasks

[yohamta]

Currently, accessing DAG details (like schedule, last run time, etc.) within tasks requires workarounds or potentially undocumented environment variables. This makes it difficult for tasks to react dynamically to DAG-specific information.

We would like to add a new environment variable, DAG_DETAILS_JSON, which will contain a JSON representation of all relevant DAG metadata. This will provide a consistent and comprehensive way for tasks to access this information, especially useful for executors like jq that can easily parse JSON.

Example Usage (with jq):

steps:
  - name: get schedule
    executor: jq
    command: '.schedule'
    script: "echo $DAG_DETAILS_JSON"
    output: DAG_SCHEDULE

Related discussions: #708