Expose CancelRequested boolean in WorkflowExecutionInfo

[carlydf]

What changed? Add CancelRequested boolean to WorkflowExecutionInfo message

Because there was previously no way to see whether a cancellation was requested, even though the server already tracks that internally. You could only see whether the cancel had finished via the WorkflowExecutionStatus, but that left the case where the cancel was requested but not completed ambiguous.

Fixes https://github.com/temporalio/temporal/issues/1018

I don't think so, since it's a new field, not a removed field. If it's blank, the value will just be false.