Add proxy support

[mrodm]

In this Pull Request, it's included support to include proxy support by means of adding environmetn variables in the systemd units.

These settings can be set for all instances defining them as class parameters, or they can be defined in a specific instance resource.

Additional:

• Add a new job in the github workflow to run pdk validate
  • It uses this github action
  • As this new action has been added, some changes have been performed in our files (tests, syntax, lint, ...)
  • Example of the github workflow run: https://github.com/Telefonica/puppet-github-actions-runner/actions/runs/554912288
• Added upper bounds in dependencies in metadata.son (required by pdk validate)