Feat:Compatible with Dify's External Knowledge API (#2848)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._
Fixes #2731 
### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/settings.py

@@ -250,3 +250,5 @@ class RetCode(IntEnum, CustomEnum):
     AUTHENTICATION_ERROR = 109
     UNAUTHORIZED = 401
     SERVER_ERROR = 500
+    FORBIDDEN = 403
+    NOT_FOUND = 404