feat: model.json doesn't have any required params

[0xSage]

Problem Per spec, users should only have to modify 1 field for a model to work

source_url: //techincally, the only required field

Success Criteria A clear and concise description of what you want to happen.

• Have a global default for model.json
• Maybe just a fallback via /model-extensions/src/model.json
• Maybe design a future compatible name i.e. /model-extension/src/gguf-model.json

Additional context

Once this is the case, this issue would be a breeze: https://github.com/janhq/jan/issues/678