Add endpoint to download model weights

[leoll2]

Create and implement an endpoint to allow clients to download the trained model weights in OpenVINO format.

Endpoint: GET /api/projects/{project_id}/models/{model_id}/binary

Response 200 OK + zip archive containing a model.xml and model.bin files. Preferably, set the Content-Disposition (ref) with a good file name for the zip file (e.g. model-<id>-fp16.zip.