Removed needless compilation of MockClientAPITest from the client library and also renamed the unit_tests directory to just test

The intent log exists in the root entry only for the agent register creation. Then the agent register has an agent creation intent log
as well. And finally for all other usages, the agent's intent log shall be used.

This is a working compilation checkpoint. Some intent logs still need to be removed from the schema.

Some legacy utilities from castor were imported alongside, with some adaptations.

    CTA/libs/client
    CTA/libs/common
    CTA/xroot_clients
    CTA/xroot_plugins