Improve storage engine (#4341)

### What problem does this PR solve?

- Bring `STORAGE_IMPL` back in `rag/svr/cache_file_svr.py`
- Simplify storage connection when working with AWS S3

### Type of change

- [x] Refactoring

conf/service_conf.yaml

@@ -22,7 +22,7 @@ infinity:
   db_name: 'default_db'
 redis:
   db: 1
-  password: 'infini_rag_flow' 
+  password: 'infini_rag_flow'
   host: 'redis:6379'
 
 # postgres:
@@ -34,7 +34,6 @@ redis:
 #   max_connections: 100
 #   stale_timeout: 30
 # s3:
-#   endpoint: 'endpoint'
 #   access_key: 'access_key'
 #   secret_key: 'secret_key'
 #   region: 'region'