Create factorial_iterative.py

[agarwalpranay02]

Describe your change

• Add an algorithm

Created a new Python file factorial_iterative.py implementing the factorial function using an iterative approach. This algorithm was not present earlier. Included type hints and a Wikipedia reference as required.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] This pull request is all my own work and I have not plagiarized.
• [x] I know this PR will not be merged if it fails the automated tests.
• [x] This PR only changes one algorithm file.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with type hints.
• [x] All functions have doctrings and pass automated testing.
• [x] Includes at least one URL reference to explanation.