feat: Enhance crawler flexibility and LLM extraction capabilities

- Add browser type selection (Chromium, Firefox, WebKit)
- Implement iframe content extraction
- Improve image processing and dimension updates
- Add custom headers support in AsyncPlaywrightCrawlerStrategy
- Enhance delayed content retrieval with new parameter
- Optimize HTML sanitization and Markdown conversion
- Update examples in quickstart_async.py for new features

crawl4ai/web_crawler.py

@@ -12,6 +12,7 @@ from typing import List
 from concurrent.futures import ThreadPoolExecutor
 from .config import *
 import warnings
+import json
 warnings.filterwarnings("ignore", message='Field "model_name" has conflict with protected namespace "model_".')