FTP Probe

Because not all FTP servers are accessable through http, a real FTP probe is necessary to check these FTP servers.

Checks to perform are:
- is the ftp server alive
- can I login with the given user
- is a given directory available
- is a given file in a given directory available

For our use case it is important to know whether an external FTP server is still alive and kicking and the directory where we download data from is available.

Start by locating the existing probe or health-check implementations and their tests. Read how checks receive connection details and report status, then determine where an FTP probe belongs. Done means it can verify server availability, login credentials, directory availability, and file availability for the configured external FTP server.

Written by the indexing model from the issue text.