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

[teodorkostov]

Pointless. Mount FTP to file system and run rqbit.

This PR will increase the codebase needlessly.

[milahu]

Mount FTP to file system and run rqbit

this should run on a shared server, so i have:
no root access, no fuse kernel module, no ptrace, ...

the first step to implement such a network storage backend would be
to convert all filesystem operations to async

[teodorkostov]

If you are using it like that and you are facing such challenges it should occur to you that the technologies you are using are probably not right for the job. Storage for servers, VMs and orchestrators is a problem that has been already solved so many times. No point in adding code to a torrent client that would solve the same thing again.