Enhance privacy preservation

[nikita-kotsehub]

Currently, endpoints send back the model weights, which isn't secure. Here some ideas we can implement:

1. Send back just the updates
2. Encode the position of the model updates and send back only the weights that changed (assuming only a few weights change)
3. Add a mask to each individual model update such that upon aggregate the masks are negated
4. Add encryption to the weights before they enter the funcX pipeline