feat: improve metadata handling in connector service (#11421)

### What problem does this PR solve?

- Update sync data source to handle metadata properly

### Type of change

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

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Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

common/data_source/models.py

@@ -94,6 +94,7 @@ class Document(BaseModel):
     blob: bytes
     doc_updated_at: datetime
     size_bytes: int
+    metadata: Optional[dict[str, Any]] = None
 
 
 class BasicExpertInfo(BaseModel):