refactor(proxy): move ProxyConfig to async_configs and improve LLM token handling

Moved ProxyConfig class from proxy_strategy.py to async_configs.py for better organization.
Improved LLM token handling with new PROVIDER_MODELS_PREFIXES.
Added test cases for deep crawling and proxy rotation.
Removed docker_config from BrowserConfig as it's handled separately.

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

docs/md_v2/blog/releases/0.5.0.md

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