use sftp.NewRequestServer with custom handlers

[AkihiroSuda]

https://github.com/AkihiroSuda/sshocker/blob/3144baeef5209e6cb6428a0cd48aa8fb26a2218e/pkg/reversesshfs/reversesshfs.go#L74-L81

We should use https://pkg.go.dev/github.com/pkg/[email protected]#NewRequestServer to prevent the remote host from accessing unexpected local directories (if sshfs had a bug)