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/config.py

@@ -40,6 +40,7 @@ class BlobType(str, Enum):
 
 class DocumentSource(str, Enum):
     """Document sources"""
+    RSS = "rss"
     S3 = "s3"
     NOTION = "notion"
     R2 = "r2"