APIs for cancelling WorkflowExecution

[nonplus]

I'd like access to APIs for cancelling a WorkflowExecution (i.e. requestCancelWorkflowExecution) and to test the EventList whether a cancel has been requested (i.e. a WorkflowExecutionCancelRequested event has been recorded).

Specifically, I'd hope to see the following:

• WorkflowExecution.requestCancel() method
• EventList.requested_cancel_workflow() method (though not sure about the name)

[KleinYuan]

@nonplus
I think there's such a function in the lib/decision-response:
request_cancel_external_workflow
cancel_workflow