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 e9dcd1697cb8cbeaa4a4dd6c7c349d61a79955cf

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
  • https://github.com/jpillora/cloud-torrent/issues/24