Implemented FastAPI Server with Message, Mention, and File Event Handlers

[fumedev]

ADDED

• Added a main.py file in the src directory to set up the main Uvicorn server for the application.
• Added an app.py file in the src directory to create the ASGI application setup using FastAPI, including routes for message, mention, and file events handling.
• Added a message_handler.py file in the api_handlers directory to implement the message handling module with a placeholder for the actual logic.
• Added a mention_handler.py file in the api_handlers directory to implement the mention handling module with a placeholder for the actual logic.
• Added a file_handler.py file in the api_handlers directory to implement the file handling module with a placeholder for the actual logic.
• Added an openai_integration.py file in the ai_integration directory to implement the AI integration module with a method to process conversations using OpenAI and returning a generated response.