Split the middle-tier API into MiddleTierAdmin and MiddleTierUser

CMakeLists.txt

libs/client/CMakeLists.txt

libs/client/MiddleTierAdmin.hpp

libs/client/MiddleTierUser.cpp

libs/client/MiddleTierUser.hpp

libs/client/MockMiddleTierAdmin.cpp

libs/client/MockMiddleTierAdmin.hpp

libs/client/MockMiddleTierAdminTest.cpp

libs/client/MockMiddleTierDatabase.cpp

libs/client/MockMiddleTierDatabase.hpp

libs/client/MockMiddleTierUser.cpp

libs/client/MockMiddleTierUser.hpp

libs/client/MockMiddleTierUserTest.cpp

libs/client/TestingMockMiddleTierAdmin.hpp

libs/client/UtilTest.cpp

libs/client/Utils.cpp

libs/client/Utils.hpp

libs/client/UtilsTest.cpp

tests/CMakeLists.txt

xroot_plugins/XrdProFilesystem.cpp