Refactor desired_config_generator.rb to Builder pattern

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183632



Merged-by: Doug Stull <dstull@gitlab.com>
Approved-by: Daniyal Arshad <darshad@gitlab.com>
Approved-by: Doug Stull <dstull@gitlab.com>
Reviewed-by: Chad Woolley <cwoolley@gitlab.com>
Reviewed-by: Daniyal Arshad <darshad@gitlab.com>
Co-authored-by: Chad Woolley <cwoolley@gitlab.com>