Add an "optional" field for CMake projects

Currently the script will stop with an error if the CMakeUserPresets.json file can not be written in the output_path of some CMake project. If the additional field "optional" would be set to True then the generator would just skip that project and move on the next ones.

Motivation : allows for instance the user_options.json of Proxy-egos to declare a project for TPL, and the generator would still work in the case didn't install TPL mirror because he prefers installing dependencies using Guix or Spack.