Move singleton to common directory (#10935)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/opendal_conn.py

@@ -4,7 +4,7 @@ import pymysql
 from urllib.parse import quote_plus
 
 from api.utils.configs import get_base_config
-from rag.utils import singleton
+from common.decorator import singleton
 
 
 CREATE_TABLE_SQL = """