feat: added flight number extraction and save to JSON param

[atharvvvg]

1. Flight Number Extraction:

• Updated the core scraping logic in fast_flights/core.py to parse the data-travelimpactmodelwebsiteurl attribute.
• Successfully extracts flight codes for both non-stop (e.g., QP-1339) and multi-stop itineraries (e.g., IX-1756, AI-2466).
• Added a flight_code field to the Flight schema.

2. Save to JSON File:

• Added an --output_file argument to example.py.
• Users can now specify a file path to save the flight search results directly as a JSON file, streamlining data collection.

3. One-Way Flight Support:

• The example.py script was also updated to support one-way searches by making the --return_date argument optional.

[AWeirdDev]

oops... the new version is actually in the dev branch, the main one is actually legacy code :(