Added callback_file & callback_name to default_args DAG level and tests

[subbota19]

Hi astro team!

This PR brings the following enhancements:

1. Unittest for DAG-Level Callback using parameters:

on_success_callback_name & on_success_callback_file
on_failure_callback_name & on_failure_callback_file

Astro instruction on how to apply these parameters

2. While I find the existing callback feature useful which allows specifying callback code at any location, this PR takes it a step further by enabling callbacks to be specified within the DAG's default_args. With this enhancement, the callbacks will automatically propagate to the task level as well. This update aligns with how default_args are passed in Airflow. Additionally, I’ve included unit tests.

[tatiana]

This is super helpful, @subbota19 , thank you very much for working on this!
I'm happy for us to merge this change once we confirm the tests are passing.

[subbota19]

Hi @tatiana !
I've added another commit to address the failed fmt-check step. The code is now formatted correctly, could we please rerun the workflow one more time?