feat: Engine - New Cortex extensions supported with 3 new engines

[Van-QA]

• Cortex as extension -> 3 engine along side with Nitro => allow Jan calling C‌ortex API (only chat completion)
  • Llamacpp: bundle with jan app, download by default
  • Onnx and tensorRT => downloading together with specific model download
• Nitro.llamacpp version: keep locked version, remain unchanged