Ability to Cancel a running workflow

[andrewbroberg]

Just wondering if this would be a nice feature. The idea would be that you could cancel a running workflow if it is running or waiting status.

My high level thinking would be once the workflow is cancelled, the next time the Workflow is executed it would detect the state as cancelled and bail. I assume we'd also need to have any Activity or Signal that is currently running, once it attempts it's next try it should check if the workflow is still active, and if not it should not execute.

Happy to take an attempt at a PR for this.