refactor(proxy): consolidate proxy configuration handling

Moves ProxyConfig from configs/ directory into proxy_strategy.py to improve code organization and reduce fragmentation. Updates all imports and type hints to reflect the new location.

Key changes:
- Moved ProxyConfig class from configs/proxy_config.py to proxy_strategy.py
- Updated type hints in async_configs.py to support ProxyConfig
- Fixed proxy configuration handling in browser_manager.py
- Updated documentation and examples to use new import path

BREAKING CHANGE: ProxyConfig import path has changed from crawl4ai.configs to crawl4ai.proxy_strategy

docs/md_v2/blog/releases/0.5.0.md

@@ -251,7 +251,7 @@ from crawl4ai import (
     RoundRobinProxyStrategy,
 )
 import asyncio
-from crawl4ai.configs import ProxyConfig
+from crawl4ai.proxy_strategy import ProxyConfig
 async def main():
     # Load proxies and create rotation strategy
     proxies = ProxyConfig.from_env()