Allow listing of current cta-frontend configuration with a dedicated command

Problem to solve

Operators have all information they need for cta-taped processes, but nothing allow them to understand cta-frontend configuration.

This becomes more important as our new operation model with separate shedulerDB backends for repack and production will require to deploy more frontends in production and these will need to be more closely monitored.

Stakeholders

CTA operators.

Proposal

Basically we need the same information as the dr ls command for all the cta-frontend configuration file entries.

This should also include the CTA software version, just like it is presented in the dr ls command.

{
  "ctaVersion": "4.10.8-1",
  "frontendConfig": [
    {
      "category": "general",
      "key": "cta.repack.repack_max_files_to_select",
      "value": "200000",
      "source": "/etc/cta/cta-frontend-xrootd.conf:8"
    },
...
}

This superseedes #58 (closed)