Add FTP endpoint pair

[npofsi]

go-service-ftp now use tcp:// protocol to access to file server which is less precise.
I proposal to add an endpoint pair ftp:// like tcp://, which uses same way to obtain host and port

[Xuanwo]

For now, I don't think we need an ftp protocol. FTP is a protocol based on TCP, so we will use tcp as an endpoint to connect an FTP server. The real problem we meet here is ftp protocol only supports TCP, and itself is a widely used protocol.

Maybe we need to support default protocol for endpoint:

Turn ftp://?endpoint=tcp:127.0.0.1:1080 into ftp://?endpoint=127.0.0.1:1080.