Add inventory functions

include/agent/inventory.h

include/agent/linux/get_macaddr.h

include/agent/linux/get_machine_type.h

include/agent/linux/get_user_count.h

include/agent/linux/get_users_count.h

include/agent/windows/get_macaddr.h

include/agent/windows/get_machine_type.h

src/inventory.cpp

src/linux/get_macaddr.cpp

src/linux/get_machine_type.cpp

src/linux/get_user_count.cpp

src/windows/get_macaddr.cpp

src/windows/get_machine_type.cpp