Copy classes from pluginkit which are useful for all nexus plugins

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge branch 'refs/heads/master' into NEXUS-5490-remote-user
[NEXUS-5490] nexus-rutauth-plugin (add support for REMOTE_USER header)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>