Add cloudscraper extension for proxy header support

[proxymesh]

Summary

• Add cloudscraper_proxy module with ProxyCloudScraper class
• Combine CloudScraper's CipherSuiteAdapter with proxy header support
• Preserve all CloudScraper features (Cloudflare bypass, stealth mode, TLS customization)
• Add create_scraper() as drop-in replacement for cloudscraper.create_scraper()
• Add documentation in docs/cloudscraper.rst
• Add CloudscraperTest to test harness

Usage

from python_proxy_headers.cloudscraper_proxy import create_scraper

# Drop-in replacement for cloudscraper.create_scraper()
scraper = create_scraper(
    proxy_headers={'X-ProxyMesh-Country': 'US'},
    browser='chrome'
)

scraper.proxies = {'https': 'http://proxy.example.com:8080'}
response = scraper.get('https://example.com')

# All CloudScraper features preserved
# Proxy headers sent during CONNECT

Test plan

• Run python test_proxy_headers.py cloudscraper with a proxy
• Verify proxy headers are sent during CONNECT
• Verify CloudScraper features (Cloudflare bypass) still work