feat: supported langflow api provider (#1646)

* add langflow api provider

* chore: migration

* feat: okay for non-stream req

* fix: langflow sse data extracting

* doc: add comment on langflow api

---------

Co-authored-by: Junyan Qin <rockchinq@gmail.com>

templates/default-pipeline-config.json

@@ -70,6 +70,14 @@
             "header-value": "",
             "timeout": 120,
             "output-key": "response"
+        },
+        "langflow-api": {
+            "base-url": "http://localhost:7860",
+            "api-key": "your-api-key",
+            "flow-id": "your-flow-id",
+            "input-type": "chat",
+            "output-type": "chat",
+            "tweaks": "{}"
         }
     },
     "output": {