Allow sorting files in FTPConnector

ExtendFTPConnector.listFiles to sort files based on some server-side computed attributes of files.

This uses listdir_attr method of paramiko. sort_on parameter is directly attributes of paramiko.sftp_attr.SFTPAttributes.

At the same time, adds a minimal tests, that will only run if testSFTPConnection_SFTP_URL environment variable is set to an URI of a running sftp server supporting password authentication - which means that in our current tests infrastructure it will not run.

/cc @vpelletier @aurel @rafael