Draft: CMake: FetchContent mechanism using modified variant lib and linking against variant/libUtil

Using FetchContent mechanism to get variant using the version for which I put a PR here: atlas-sw-misc/variant!1 Currently pointing to my repo; will fix back and squash when and if approved.

Moved variant download and cmake options for JSON library type and logger level to libSpec CMakeLists.txt as it makes sense for me to keep it close to the point where it is needed (going along wuth the philosophie of as much as possible self contained sub directories even within a project to keep it disentangled). But can move it back to central locations if there are different opinions; not sure what the best approach is.

In various libs variant was linked but indeed libUtil is needed/sufficient (storage.hpp mostly). Partially the libUtil header were brought in manually. I replaced that with a link agains libUtil to work out the relationships properly.