Add job.cancelWorkflow method

[jwulf]

Sometimes a worker makes a determination to cancel a workflow. With #210, the handler still needs to return a job action. A workaround is to do:

await zbc.cancelWorkflowInstance(
    job.workflowInstanceKey
)

return job.forward()

This is semantically incorrect, and a hack. The same can be said for job.complete() (which additionally would raise an error on the broker and on the client).

An alternative is to implement a job.cancelWorkflow() method that wraps this operation and can be returned like this:

return job.cancelWorkflow()