* test_k8s: introduce a DAG querying another DAG status via the Airflow API

We also introduce a custom Airflow API client class wrapping the
generated one, adding Kerberos authentication support, as all Airflow
instances running in Kubernetes have kerberos auth enabled for the API.

Signed-off-by: Balthazar Rouberol <brouberol@wikimedia.org>
Bug: T380765