[9.x] Raise ScheduledBackgroundTaskFinished event to signal when a run in background task finishes

[aaronlp]

PR #29888 added excellent new events ScheduledTaskStarting and ScheduledTaskFinished which are fired before and after a scheduled task runs.

However, where the scheduled task is set to run in background, ScheduledTaskFinished fires immediately because we push the task off to a separate process. This makes it difficult to run any relevant activities after a background task finishes, without having to hack on to the after callbacks.

This PR adds a new event ScheduledBackgroundTaskFinished which we fire within the schedule:finish command which allows us to reliably determine when a background task actually completed without having to attach after callbacks to each scheduled task.

We're adding a brand new event class and firing it without changing any additional functionality but since we're introducing a new argument to the handle method on the schedule:finish command, so targetting master branch for this.

[taylorotwell]

Personally I would just use Container::getInstance()->make(Dispatcher::class) to get the event dispatcher - that way this can be sent to 8.x as a non-breaking change.

[aaronlp]

Ok thanks. Will reopen my 8.x PR #37367 and make that change. Thanks