Add on_kill to EMR Serverless Job Operator

[dacort]

closes: #31099

This PR adds support for on_kill in EmrServerlessStartJobOperator. In the event that the job is marked as failed in the UI, it will now be cancelled in EMR Serverless as well.

Tested manually by marking a failing DAG in the UI and observing the following output in the logs and the EMR job transitioning to CANCELLED.

[2023-05-09, 20:01:59 UTC] {local_task_job_runner.py:299} WARNING - State of this instance has been externally set to failed. Terminating instance.
[2023-05-09, 20:01:59 UTC] {process_utils.py:135} INFO - Sending Signals.SIGTERM to group 1431. PIDs of all processes in the group: [1431]
[2023-05-09, 20:01:59 UTC] {process_utils.py:86} INFO - Sending the signal Signals.SIGTERM to group 1431
[2023-05-09, 20:01:59 UTC] {taskinstance.py:1585} ERROR - Received SIGTERM. Terminating subproces

Additionally, fixed a couple return types in the EMR operator.

[dacort]

Updated w/PR comments