Adding pybind11 handling into CMake config

Testing compilation locally on gitlab-registry.cern.ch/leejr/docker_exploration/cmstkph2_ci_c7:test now (which has protobuf installed). But it appears to be working.

@fravera can you try removing your local pybind11 and unsetting the relevant env vars and see if this new setup works for you to automatically grab the files and such?

Towards cms_tk_ph2/Ph2_ACF!289 (merged)

Once this compilation test works, I'll feel comfortable pushing the docker image changes to the central repo and getting it to build new central images to be picked up by the CI.