Check standalone_dag_processor config in get_airflow_health()

[bbovenzi]

Right now the UI has to check the scheduler config for standalone_dag_processor in order to determin if the dag_processor health check should be rendered.

It would be better if we checked for standalone_dag_processor in the backend and simply not return a dag_processor object as part of the public/health endpoint if False

[vatsrahul1001]

@bbovenzi would like to work on this. Can I assign it to myself?

[bbovenzi]

@vatsrahul1001 all yours!