Add ScheduledEventID to NexusOperationCancelRequest[Completed|Failed] (#572)

**What changed?**
Added ScheduledEventID of the Nexus operation to the new
NexusOperationCancelRequestCompleted and
NexusOperationCancelRequestFailed events

**Why?**
For efficient lookups. Go SDK stores cancellation state machines keyed
on scheduled event ID.

**Server PR**
temporalio/temporal#7612

Author: pdoerner