feat(core): Support higher-order operators (#1984)

Co-authored-by: 谨欣 <echo.cmy@antgroup.com>

dbgpt/serve/agent/resource/knowledge.py

@@ -64,6 +64,7 @@ class KnowledgeSpaceRetrieverResource(RetrieverResource):
     """Knowledge Space retriever resource."""
 
     def __init__(self, name: str, space_name: str, context: Optional[dict] = None):
+        # TODO: Build the retriever in a thread pool, it will block the event loop
         retriever = KnowledgeSpaceRetriever(
             space_id=space_name,
             top_k=context.get("top_k", None) if context else 4,