Support Graceful Termination of Orchestrations and Activities using CancellationToken

Similar to #565 , it would be greate if TaskActivities/TaskOrchestrations could be dispatched with a developer-provided cancellation token.

For example, if we could provide a cancellation token as part of `StartAsync` on a single worker node, that can be propagated to dispatched orchestrations/activities to allow developers have more control over termination of running activities/orchestrations.

```c#
taskHubWorker.StartAsync(cancellationToken);
```

An example use case would be interface activities:

```c#
public interface IInterfaceTask
{
Task WorkAsync(CancellationToken cancellationToken);
}

public class Orchestration : TaskOrchestration
{
public Task RunTask(OrchestrationContext context, bool input)
{
var interfaceTask = context.CreateClient();
await interfaceTask.WorkAsync(context.CancellationToken);
}
}
```