Fixes#7650
This change enables the SFTP server to reload the user store configuration
(sftp_userstore.json) when a HUP signal is sent to the process, without
requiring a service restart.
Changes:
- Add Reload() method to FileStore to re-read users from disk
- Add Reload() method to SFTPService to handle reload requests
- Register reload hook with grace.OnReload() in sftp command
This allows administrators to add users or change access policies
dynamically by editing the user store file and sending a HUP signal
(e.g., 'systemctl reload seaweedfs' or 'kill -HUP <pid>').
