Rewrite the FTP-related plugins under embulk.org

[dmikurube]

The FTP-related plugins under embulk.org (such as embulk-input-ftp) uses the FTP4J, but it has no longer been maintained since its v1.7.1 in 2012.

https://www.sauronsoftware.it/projects/ftp4j/previous.php

We need to migrate them to some other method for FTP, but it is expected to be a "total rewrite".

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We are looking for a great contributor for this upgrade, which effectively means that we are looking for a new maintainer for the long-term.

For example, Apache Commons Net could be an alternative to FTP4J, but it's up to the new maintainer.

If you're interested in it, please let us know in this Discussion at first.

[dmikurube]

Related issues:

• Reimplement embulk-input-ftp with Apache Commons-Net, not FTP4J embulk-input-ftp#29