Add FastAPI example main.py witha simple GET endpoint

[mimionly]

Added main.py as a simple FastAPI example demonstrating a basic GET endpoint.

The application defines a single route ("/") that returns a JSON message:
{
"message": "Hello, FastAPI!"
}

This serves as a quick-start example for beginners to understand how to:

• Initialize a FastAPI app
• Define a GET endpoint
• Run it locally using Uvicorn

To run this example:

1. Install dependencies:
   pip install fastapi uvicorn

2. Run the app:
   uvicorn main:app --reload

3. Open in your browser:
   http://127.0.0.1:8000/

[vinta]

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