Pass/get variables with $CI_JOB_TOKEN pipeline triggers

What does this MR do?

This MR allows user to pass variables over cros-project pipelines. Formerly, this was unavailable, because we didn't have a room for persisting variables on a pipeline. We just implemented the new architecture Ci::PipelineVariables and finally reached to the goal.

Are there points in the code the reviewer needs to double check?

Does spec/services/ci/pipeline_trigger_service_spec.rb test the feature?

Why was this MR needed?

This is Deliverable for %9.5. Originally requested at https://gitlab.com/gitlab-org/gitlab-ee/issues/2772.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ee/issues/2772