Added cta::SecurityIdentity

libs/client/CMakeLists.txt

libs/client/ClientAPI.hpp

libs/client/FileSystemNode.cpp

libs/client/FileSystemNode.hpp

libs/client/MockClientAPI.cpp

libs/client/MockClientAPI.hpp

libs/client/MockClientAPITest.cpp

libs/client/SecurityIdentity.cpp

libs/client/SecurityIdentity.hpp

libs/client/TestingMockClientAPI.hpp

libs/client/UserIdentity.cpp

libs/client/UserIdentity.hpp

xroot_plugins/XrdProFilesystem.cpp

xroot_plugins/XrdProFilesystem.hpp