AIP-44 Migrate DagFileProcessorManager._fetch_callbacks to Internal API

[mhenc]

https://github.com/apache/airflow/blob/main/airflow/dag_processing/manager.py#L663

[bugraoz93]

Hey,
I would like to work on this issue, please.

[mhenc]

Can you sync with @vincbeck as he may have covered that part as well:
#28900

[vincbeck]

Yeah I think I will include it in #28900

[bugraoz93]

Thanks a lot for the update! You are fast :) Let's assign the issue to @vincbeck