A button to cancel workflow along with terminate

[longquanzheng]

Right now only `terminate` is supported. In some use cases cancel is needed.

As a workaround, users have to use CLI to cancel their workflows.

[petergardfjall]

Any plans to address this? Seems like a logical (?) and simple feature to implement. Termination provides no means for the code to properly clean up. As an example, I discovered that goroutines would leak for terminated workflows awaiting child workflows with ChildWorkflowFuture.Get() so Cancel is probably preferable in most circumstances and Terminate would only be a last resort. Much like kill -INT might be tried prior to a kill -KILL.