Add cpm, update gitlab-ci.yml

Replace ExternalProject with CPM as dependency management, in this also move all fetching of packages to the root CMakeLists.txt.

The search of installation should be easier now with a custom Findelsa.cmake as well, which searches for the include directory and the configuration files of the installed elsa version.

Flatten folder structure to make the structure a little less weird.

Update docker images in CI, update clang-format script