Store incoming, unprocessed cargo in object store

[gnarea]

Describe the problem

We're currently storing incoming cargoes in NATS Streaming, which internally stores it in PostgreSQL. This is bad for performance, reliability and cost reasons.

Describe the solution you'd like

Store incoming cargo in the object store (e.g., GCS in production) and use NATS Streaming only to notify about the reception of each cargo, just like we do with parcels.