Add pycurl extension for proxy header support

[proxymesh]

Summary

• Add pycurl_proxy module with ProxyCurl class wrapping pycurl
• Support sending custom headers to proxy via CURLOPT_PROXYHEADER
• Capture proxy CONNECT response headers via HEADERFUNCTION callback
• Add convenience functions (get, post, etc.) matching existing API style
• Add documentation in docs/pycurl.rst
• Add PycurlTest to test harness

Usage

from python_proxy_headers.pycurl_proxy import ProxyCurl, get

# Using the class
curl = ProxyCurl(proxy_headers={'X-ProxyMesh-Country': 'US'})
response = curl.get('https://example.com', proxy='http://proxy:8080')
print(response.proxy_headers)  # Headers from proxy CONNECT response

# Using convenience function  
response = get('https://example.com',
               proxy='http://proxy:8080',
               proxy_headers={'X-Custom': 'value'})

Test plan

• Run python test_proxy_headers.py pycurl with a proxy that returns X-ProxyMesh-IP
• Verify proxy headers are sent during CONNECT
• Verify proxy response headers are captured