feat(proxy): add proxy rotation strategy

Implements a new proxy rotation system with the following changes:
- Add ProxyRotationStrategy abstract base class
- Add RoundRobinProxyStrategy concrete implementation
- Integrate proxy rotation with AsyncWebCrawler
- Add proxy_rotation_strategy parameter to CrawlerRunConfig
- Add example script demonstrating proxy rotation usage
- Remove deprecated synchronous WebCrawler code
- Clean up rate limiting documentation

BREAKING CHANGE: Removed synchronous WebCrawler support and related rate limiting configurations

crawl4ai/__init__.py

@@ -8,6 +8,10 @@ from .content_scraping_strategy import (
     WebScrapingStrategy,
     LXMLWebScrapingStrategy,
 )
+from .proxy_strategy import (
+    ProxyRotationStrategy,
+    RoundRobinProxyStrategy,
+)
 from .extraction_strategy import (
     ExtractionStrategy,
     LLMExtractionStrategy,
@@ -60,31 +64,33 @@ __all__ = [
     "DisplayMode",
     "MarkdownGenerationResult",
     "Crawl4aiDockerClient",
+    "ProxyRotationStrategy",
+    "RoundRobinProxyStrategy",
 ]
 
 
-def is_sync_version_installed():
-    try:
-        import selenium # noqa
+# def is_sync_version_installed():
+#     try:
+#         import selenium # noqa
 
-        return True
-    except ImportError:
-        return False
+#         return True
+#     except ImportError:
+#         return False
 
 
-if is_sync_version_installed():
-    try:
-        from .web_crawler import WebCrawler
+# if is_sync_version_installed():
+#     try:
+#         from .web_crawler import WebCrawler
 
-        __all__.append("WebCrawler")
-    except ImportError:
-        print(
-            "Warning: Failed to import WebCrawler even though selenium is installed. This might be due to other missing dependencies."
-        )
-else:
-    WebCrawler = None
-    # import warnings
-    # print("Warning: Synchronous WebCrawler is not available. Install crawl4ai[sync] for synchronous support. However, please note that the synchronous version will be deprecated soon.")
+#         __all__.append("WebCrawler")
+#     except ImportError:
+#         print(
+#             "Warning: Failed to import WebCrawler even though selenium is installed. This might be due to other missing dependencies."
+#         )
+# else:
+#     WebCrawler = None
+#     # import warnings
+#     # print("Warning: Synchronous WebCrawler is not available. Install crawl4ai[sync] for synchronous support. However, please note that the synchronous version will be deprecated soon.")
 
 # Disable all Pydantic warnings
 warnings.filterwarnings("ignore", module="pydantic")