Factorized out the cta-xroot_plugins-write-notification-msg command

xroot_plugins/CMakeLists.txt

xroot_plugins/FakeEosCmd.cpp

xroot_plugins/FakeEosCmd.hpp

xroot_plugins/WriteNotificationMsgCmd.cpp

xroot_plugins/WriteNotificationMsgCmd.hpp

xroot_plugins/WriteNotificationMsgCmdLineArgs.cpp

xroot_plugins/WriteNotificationMsgCmdLineArgs.hpp

xroot_plugins/WriteNotificationMsgCmdMain.cpp