[DISCUSS] Rubocop setups for Ruby styleguide

This pull request contains:
- a standard Rubocop configuration with the aim to use it for all GDS Ruby projects
- updates to the Ruby styleguide linking each individual Rubocop rule to their corresponding rules in the GDS styleguide

The cops in the gds-ruby-styleguide.yml file are all the rules explicitly defined in the styleguide, enabled by default. 
The other yml files specify all other cops available with Rubocop, disabled by default.