[FEATURE REQUEST]: Support for OpenAI function API

[stoyan-stoyanov]

The problem you are trying to solve:

I want the assistants I build to be able to call functions based on the contents in a conversation. OpenAI's has released a new native function-calling capabilities but they are currently not supported in LLMFlows.

Suggested new feature or change:

I would like to have support for OpenAI's function calling in OpenAIChat, ChatFlowStep, and AsyncChatFlowStep so I can integrate function calling in flows.