Add test harness for proxy header extensions

- Add test_proxy_headers.py script to test all 4 extensions
- Support configuration via environment variables (PROXY_URL, TEST_URL, PROXY_HEADER)
- Modular design with base ModuleTest class for easy extension
- Test urllib3, requests, aiohttp, and httpx modules
- Reports pass/fail status and header values

Usage:
  export PROXY_URL='http://user:pass@proxy:8080'
  python test_proxy_headers.py [module1] [module2] ...

Author: Cursor