feature: Chat completion functions #588

Have a feature that mimics OpenAI functions: https://openai.com/blog/function-calling-and-other-api-updates.

From the documentation, it seems OpenAI trained the model to respond to the system prompt.

For LocalAI, I think there should be a minimum viable implementation that should work across models. While custom models could be trained to reflect a specific prompt that is injected, that would limit this functionalities to restricted models.