feat: enhance model/sources APIs

[louis-menlo]

Discussion issues:

janhq/jan#4170

As discussed, there are a few missing pieces of information that we need to update the endpoint.

• Model description (README.md?)
• Rate limiting? Users would not want to put their huggingface token
• Sibling file size
• Total download (persist on adding only?)

Out of scope:

• Likes update (persist on adding only?)
• Modality
• Languages (model tags?)
• Comparison (MLU / Speed)

[louis-menlo]

Regarding the rate-limiting issue, the client application will still initialize a list of models to provide a better user experience.

So that:

1. As soon as users open the app, they'll head straight to the Hub, where they'll immediately see a list of popular models without having to wait for the Hub to update.
2. We avoid rate limiting because we don't allow clients to pull multiple models simultaneously.
3. The hub should be updated gradually or implemented with a smart searching mechanism.