Cancel background jobs

## Feature Request

### Problem Overview
Cancelling workflow by Elsa Studio or WorkflowExecutionContext.Cancel doesn't cancel CancellationToken in activities with attribute [Activity(Kind = ActivityKind.Job)]

### Proposed Solution
Cancelling workflow by Elsa Studio or WorkflowExecutionContext.Cancel should cancel CancellationToken in all nested activities or workflows.

### Alternative Solutions
I could cancel BackgroundJobs by adding NotificationHandler:
```
public class WorkflowCancelingNotificationHandler : INotificationHandler
{
private readonly IWorkflowRuntime workflowRuntime;
private readonly IBackgroundActivityScheduler backgroundActivityScheduler;

public WorkflowCancelingNotificationHandler(
IWorkflowRuntime workflowRuntime,
IBackgroundActivityScheduler backgroundActivityScheduler)
{
this.workflowRuntime = workflowRuntime;
this.backgroundActivityScheduler = backgroundActivityScheduler;
}

public async Task HandleAsync(WorkflowCancelling notification, CancellationToken cancellationToken)
{
var client = await workflowRuntime.CreateClientAsync(notification.WorkflowInstanceId, cancellationToken);
var workflowState = await client.ExportStateAsync(cancellationToken);
var stimuluses = workflowState.Bookmarks
.Select(b => b.Payload)
.OfType()
.ToList();
foreach (var stimulus in stimuluses)
{
if (string.IsNullOrEmpty(stimulus.JobId))
{
continue;
}
await backgroundActivityScheduler.CancelAsync(stimulus.JobId);
}
}
}
```
WorkflowExecutionContextExtension:
```
public static class WorkflowExecutionContextExtension
{
public static async Task CancelAll(this WorkflowExecutionContext context)
{
var service = context.GetRequiredService();
await service.CancelWorkflowAsync(context.Id);
context.Cancel();
}
}
```
and register it:
```
services.AddNotificationHandler();
```

### Use Cases
It maybe useful to prevent duplicate of long-running jobs, when job must be singleton with timeout. It's not intuitive now, when workflow's sub status is cancelled, but some long task inside activity doesn't.