Draft: Add cop Gitlab/RoutingUrlHelpers

Description of the proposal

This MR adds a new 👮 Gitlab/RoutingUrlHelpers to enforce use of Gitlab::Routing.url_helpers instead of Rails.application.routes.url_helpers.

This MR also generates TODOs for Gitlab/RoutingUrlHelpers and enable "grace period" and adds a new section URL helpers to development guidelines in https://docs.gitlab.com/ee/development/routing.html.

Why?

Gitlab::Routing implements more convenient path helpers which are not available via Rails.application.routes.url_helpers.

Code

# bad
Rails.application.routes.url_helpers.root_path(foo: :bar)

# good
Gitlab::Routing.url_helpers.root_path

Example offense

ee/app/helpers/projects/security/api_fuzzing_configuration_helper.rb:8:33: C: [Correctable] Gitlab/RoutingUrlHelpers: Prefer Gitlab::Routing over Rails.application.routes. See https://docs.gitlab.com/ee/development/routing.html
      gitlab_ci_yaml_edit_path: Rails.application.routes.url_helpers.project_ci_pipeline_editor_path(project),
                                ^^^^^^^^^^^^^^^^^^^^^^^^

Check-list

• Make sure this MR enables a static analysis check rule for new usage but ignores current offenses.
• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend).
  • Vote for both choices, so they are visible to others.
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus).
• Create a follow-up issue to fix the current offenses as a separate iteration: #390359 (closed)
• Follow the review process as usual.
• Once approved and merged by a maintainer, mention it again:
  • In the relevant Slack channels (e.g. #development, #backend, #frontend).
  • (Optional depending on the impact of the change) In the Engineering Week in Review.

/cc @gitlab-org/maintainers/rails-backend