Extend Airflow operator by implementing CeleryKubernetesExecutor

[adwk67]

As a user I want to have the option of running my airflow DAGs with the CeleryKubernetesExecutor, so that I have greater control over resource configuration across static workers (jobs queued via Redis) combined with dynamic works.

CeleryKubernetesExecutor

this combines a fixed number of celery workers plus the ability to spin up extra pods if the celery queue is full
this requires the KubernetesExecutor to be configured
The full list is here: airflow.apache.org/docs/apache-airflow/stable/executor/index.html#executor-types

Related to #2.