feat(lightrag): 添加 查询时使用embedding缓存功能

- 在 LightRAG 类中添加 embedding_cache_config配置项
- 实现基于 embedding 相似度的缓存查询和存储
- 添加量化和反量化函数，用于压缩 embedding 数据
- 新增示例演示 embedding 缓存的使用

README.md

@@ -596,6 +596,7 @@ if __name__ == "__main__":
 | **enable\_llm\_cache** | `bool` | If `TRUE`, stores LLM results in cache; repeated prompts return cached responses | `TRUE` |
 | **addon\_params** | `dict` | Additional parameters, e.g., `{"example_number": 1, "language": "Simplified Chinese"}`: sets example limit and output language | `example_number: all examples, language: English` |
 | **convert\_response\_to\_json\_func** | `callable` | Not used | `convert_response_to_json` |
+| **embedding\_cache\_config** | `dict` | Configuration for embedding cache. Includes `enabled` (bool) to toggle cache and `similarity_threshold` (float) for cache retrieval | `{"enabled": False, "similarity_threshold": 0.95}` |
 
 ## API Server Implementation