scheduler: route task cancellation through callback hook

[phillipleblanc]

Summary

• add a scheduler config callback type (OnCancelTasksFn) for cancelling running tasks per executor
• update ExecutorManager::cancel_running_tasks to invoke the callback when configured, with fallback to existing gRPC cancellation behavior
• add a unit test to verify cancellation requests are grouped by executor and dispatched via the callback

Task cancellation was previously only supported through the ExecutorGrpc method, this allows the hosting application more flexibility when deciding how to cancel tasks.