fsapp: implement app.inspect based `iter_active`

[pmrowla]

Currently we implement iter_queued and iter_processed for manually accessing tasks as raw messages via the FS kombu broker. (This is needed for when there is no worker process running, since celery.control.inspect requires talking to a worker via broadcast messages)

https://github.com/iterative/dvc-task/blob/5a49fbe660d6567c0153f217ff3755bda4e78c4a/src/dvc_task/app/filesystem.py#L123-L130

Ideally we should also have an inspect-based wrapper which can return actual active tasks via celery.inspect for use when we have an active worker process