Add a proper CLI utility

[LecrisUT]

This would help with the tests and adding simple smoke tests. Something like example/json-schema-validate.cpp might be ok, but it would be better if there is a proper one that can parse --version and such flags. I can design a simple one using CLI11 with FetchContent if that is ok with you.

[pboettch]

I have my own app/option-parser lib (cxx_args). But cli11 looks superior. Do not hesitate to suggest a change by using CLI11.