Add Filesystem as a backend for CeleryManager in background callbacks

How is this feature useful?

• When the value that the background callback returns is bigger than 512MB (formatted as a string), an error will be raised: celery.exceptions.BackendStoreError: value too large for Redis backend
• This error occurs because the CeleryManager temporarily stores the output value of the background callback.
• A workaround for this could be using a FilesystemBackend (for example, the DE5 Persistent Filesystem), like this:

celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend='../mount')
background_callback_manager = CeleryManager(celery_app)

What's the current state?

• In the CeleryManager for background callbacks, the values are stored as strings (ref code: link).
• When the backend is a filesystem, celery (as a dependency of the CeleryManager) expects the format to be bytes (ref: link).
• The current version of dash CeleryManager for background callbacks doesn't contemplate the case where the backend is a filesystem; it assumes Redis will be used as a backend (ref: link).
• Celery does contemplate the option os using a FilesystemBackend (ref link, but the CeleryManager doesn't know how to handle that situation.

What changes would be necessary?

• In places where cache.set(progress_key, json.dumps(progress_value, cls=PlotlyJSONEncoder)) is used directly add a conditional logic to support both RedisBackend and FilesystemBackend like:

from kombu.utils.encoding import str_to_bytes [[source]](https://docs.celeryq.dev/projects/kombu/en/latest/_modules/kombu/utils/encoding.html#str_to_bytes)
...

if isinstance(celery_app.backend, RedisBackend):
    cache.set(progress_key, json.dumps(progress_value, cls=PlotlyJSONEncoder))
elif isinstance(celery_app.backend, FilesystemBackend):
    celery_app.backend.set(progress_key, str_to_bytes(json.dumps(progress_value(progress_value)))
    # ref: https://github.com/celery/celery/blob/main/celery/backends/filesystem.py#L87

• If necessary - add some logic that detects if the value passed to the backend is a Redis database or a filepath > It doesn't seem to be necessary since Celery detects this automatically: https://github.com/celery/celery/blob/main/celery/app/base.py#L1092

Start in dash/long_callback/managers/celery_manager.py, especially the direct cache.set calls described in the issue. Compare the RedisBackend and FilesystemBackend behavior in the linked Celery sources; done means CeleryManager can store background-callback values with either backend without breaking the existing Redis path.