[Feature Request] Add a function to stop a running job

Hi,
It would be nice to have a `cron.stop(job_id/job_name)` to stop a running job without unscheduling it.
I assume it is the nicer equivalent of finding the pid of the job and calling `pg_cancel_backend` it (or `pg_terminate_backend if a force flag was used).

should be similar to DBMS_SCHEDULER.STOP_JOB from Oracle.

Also: I don't know if it's intended or not - but unscheduling a job using `cron.unschedule` doesn't stop it if it is running. I would assume it should. maybe related to #178

Thanks

Start by reviewing the requested cron.stop(job_id/job_name) behavior alongside the existing cron.unschedule behavior and related issue #178. Clarify whether stopping should cancel or terminate the PostgreSQL backend, how a force option should work, and whether unscheduling a running job should also stop it; done means the API and both behaviors are specified and implemented.

Written by the indexing model from the issue text.