Log the fact that a jar is being sent over channel

[Vlatombe]

This is already logged through org.jenkinsci.remoting.util.LoggingChannelListener#onJar, however since this logger also logs all packets being read or written, this produces too much noise.

With this change, setting a logger on hudson.remoting.JarLoaderImpl with level FINE will allow to capture all jars being sent over a channel.

Testing done

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