fix: add usage of `GOOGLE_API_KEY` env var (-and other improvements)

[tnvmadhav]

Fix:

• Add usage of GOOGLE_API_KEY environment variable during genai client configuration step. Fixes #2

Improvements

• Refactor for better readability by flake8 linting standards
• Refactor allowed_file function logic to make use of os.path.splitext(...) module