Serialize/deserialize Messages asynchronously

[trxcllnt]

Synchronously serializing/deserializing large Messages on the tokio worker thread blocks other asynchronous tasks.

This PR ensures Message bodies are serialized and deserialized asynchronously via spawn_blocking.

[longzhi]

The rusty-celery project seems to be no longer maintained, so I forked it and continued maintaining it here: https://github.com/GaiaNet-AI/celery-rs.

Would it be possible for you to open a PR and merge your changes into this new repository? I’d be very glad to keep this project alive with joint community maintenance.