feat: SHA/SIG files to validate the integrity of downloaded models

[LaurentBonnaud]

Problem
Sometimes, when I download a model, the download indicator remains stuck below 100%.
I usually let jan run for a long time in the hope that the download will finish anyway.
On disk, model files have the expected size.
However, I am not sure that model files have the correct content.

Success Criteria
It would be useful if jan would be able to verify the downloaded models against a known fingerprint.

Additional context
For example, ollama is able to check models against a SHA-256 fingerprint.

[marknguyen1302]

done in https://github.com/janhq/cortex/pull/986