Add time utils (#10849)

### What problem does this PR solve?

- Add time utilities and unit tests

### Type of change

- [x] Refactoring

---------

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

api/apps/system_app.py

@@ -24,7 +24,6 @@ from api.db.services.api_service import APITokenService
 from api.db.services.knowledgebase_service import KnowledgebaseService
 from api.db.services.user_service import UserTenantService
 from api import settings
-from api.utils import current_timestamp, datetime_format
 from api.utils.api_utils import (
     get_json_result,
     get_data_error_result,
@@ -32,6 +31,7 @@ from api.utils.api_utils import (
     generate_confirmation_token,
 )
 from api.versions import get_ragflow_version
+from common.time_utils import current_timestamp, datetime_format
 from rag.utils.storage_factory import STORAGE_IMPL, STORAGE_IMPL_TYPE
 from timeit import default_timer as timer