runs: Implement custom run stop logic

[yashlamba]

In the current implementation #16, we are doing celery revoke and terminate task for cancelling a run. This is ok but doesn't give us much control over cleanup and saving the progress of a task.

A custom implementation would (might) include:

• Have separate states (canceling/cancelled, stopping/stopped etc), one is updated when the stop is requested and the other for when it is actually stopped.
• Do the check in the task whether the run was requested to be stopped and do the cleanup as required.

This gives us more control over how to stop and do other transitions like pause in the future.