The Logging system should be introduced.

[ymnk]

A quote from https://github.com/ymnk/jsch-agent-proxy/commit/87cdad7a86f8e1c8ec5ef2cfc0b1b321180f8a08#commitcomment-3370774 ,

Perhaps we could allow the user to pass in an optional log callback?
I see that jsch proper has an implementation of this in its Logger class,
though we don't necessarily want to add that as a dependency.