Support function pre-execute system prompt

[adlymousa]

This PR adds support for pre-execute prompts when registering functions for OpenAILLM. These prompts are sent to the LLM before executing a function call, allowing for additional context or instructions to be provided to the user before the function executes.

For instance, we can instruct the LLM to inform the user that "we're getting the information, please hold on" before executing RAG.

[aconchillo]

Closing. This is something you can do at the beginning of each function call or you can generaically by adding an event handler for on_function_calls_started which is triggered before any function call executes.