DAG-level "on_execute_callback"

### Description

The `DAG` is missing an `on_execute_callback` argument.

### Use case/motivation

My team would like to send a notification on start of our Airflow-pipeline. We could use the task-level "on_execute_callback" of the first task, but it seems cleaner and more consistent to configure a DAG-level "on_execute_callback" for this.

Advantage: If the first task is removed/replaced, the callback does not need to be touched.

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start by locating the DAG definition and the existing task-level on_execute_callback implementation. Trace how callbacks are configured and invoked, then identify the relevant tests for DAG and task execution. Done means a DAG-level on_execute_callback can trigger a notification when the pipeline starts without depending on its first task.

Written by the indexing model from the issue text.