add FTP storage backend

[milahu]

early draft to add a FTP storage backend

use case:
bittorrent reverse proxy
running on a fast cache server with limited storage
in front of a slow backend server with large storage

remotefs-ftp does not work because remotefs is not thread-safe
see e9dcd16

async_ftp is hard to integrate
because all the storage functions are sync...
i am not good enough in Rust for such a large task
and github copilot fails on complex errors

note:
the FTP protocol is an implementation detail
it could be any network filesystem:
NFS, SMB, HTTP, WebDAV, SCP, Rsync, Rclone, ...

alternatives:

• mount the FTP server with curlftpfs or go-fuseftp, but that requires the fuse kernel module
• hook syscalls with fakechroot or proot, but these tools have no FTP storage backend
• use a different bittorrent client like libtorrent or anacrolix/torrent, and add a FTP storage backend there
  • Core Feature: Remote Storage Backends jpillora/cloud-torrent#24