Implement API endpoint for DAG deletion

[uranusjr]

Close #17969.

I’m not sure about the error returned for DagFileExists. I was considering 409 Conflict, but Airflow (and Connexion) seems to define that to only represent conflicts on POST. I’m settling for 400 Bad Request, but I feel the current definition is too narrow. The definition in RFC 7231 is much broader and IMO applies here:

The 409 (Conflict) status code indicates that the request could not be completed due to a conflict with the current state of the target resource. This code is used in situations where the user might be able to resolve the conflict and resubmit the request.

https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.8

[ephraimbuddy]

LGTM just nitpicking :)

[uranusjr]

I added an except block to catch AiflowException and return 409. The 409 description in the OpenAPI schema is also rewritten slightly to make sense for non-creation conflicts.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.