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/azure_spn_conn.py

@@ -18,7 +18,7 @@ import logging
 import os
 import time
 from rag import settings
-from rag.utils import singleton
+from common.decorator import singleton
 from azure.identity import ClientSecretCredential, AzureAuthorityHosts
 from azure.storage.filedatalake import FileSystemClient