feat: support rss datasource (#13721)

### What problem does this PR solve?

Supporting public RSS/Atom feed URLs as data sources for RagFlow.

link https://github.com/infiniflow/ragflow/issues/12313

### Type of change

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

common/data_source/__init__.py

@@ -24,6 +24,7 @@ SOFTWARE.
 """
 
 from .blob_connector import BlobStorageConnector
+from .rss_connector import RSSConnector
 from .slack_connector import SlackConnector
 from .gmail_connector import GmailConnector
 from .notion_connector import NotionConnector
@@ -55,6 +56,7 @@ from .exceptions import (
 
 __all__ = [
     "BlobStorageConnector",
+    "RSSConnector",
     "SlackConnector",
     "GmailConnector",
     "NotionConnector",