Improve storage engine

[KennyDizi]

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

• Refactoring

[JinHai-CN]

@KennyDizi Please remove the unused import statement.

rag/utils/s3_conn.py:4:29: F401 [*] `botocore.client.Config` imported but unused
  |
2 | import boto3
3 | from botocore.exceptions import ClientError
4 | from botocore.client import Config
  |                             ^^^^^^ F401
5 | import time
6 | from io import BytesIO
  |
  = help: Remove unused import: `botocore.client.Config`

[KennyDizi]

Thanks @JinHai-CN I've adjusted the code accordingly.