feat: implement Exa search engine integration

backend/open_webui/config.py

@@ -1741,6 +1741,11 @@ BING_SEARCH_V7_SUBSCRIPTION_KEY = PersistentConfig(
     os.environ.get("BING_SEARCH_V7_SUBSCRIPTION_KEY", ""),
 )
 
+EXA_API_KEY = PersistentConfig(
+    "EXA_API_KEY",
+    "rag.web.search.exa_api_key",
+    os.getenv("EXA_API_KEY", ""),
+)
 
 RAG_WEB_SEARCH_RESULT_COUNT = PersistentConfig(
     "RAG_WEB_SEARCH_RESULT_COUNT",