Refactor FileServerService: Extract file handling logic into dedicated handlers (#17090)

- Introduced FileServerDriveHandler to manage drive file requests.
- Created FileServerFileResolver for resolving file access keys and downloading files.
- Added FileServerProxyHandler to handle proxy requests and image processing.
- Moved utility functions to FileServerUtils for better organization.
- Removed redundant methods from FileServerService, improving readability and maintainability.

Author: samunohito