[FEAT] Add webhook callback after task completion

Is it possible to implement a feature where, after completion, cancellation, or failure of a DAG, the dag_json is automatically sent to a specified URL, provided when the task is launched?

No files or tests are named in the issue. Start by locating the DAG launch path and the handling for completion, cancellation, and failure; then determine how a launch-provided URL and dag_json are represented. Done should cover all three terminal outcomes and verify that the callback receives the expected payload.

Written by the indexing model from the issue text.