Follow-up from "Allow to use cross project pipelines" ver 2

What does this MR do?

This MR adds Ci::PipelineVariables model to create new room for persisting variables on pipeline. Previously, we did with trigger_request, however, it limits situations to use, so we'll switch to Ci::PipelineVariables.

Also, this MR imports relevant codes from EE as many as possible. This is for avoiding conflicts to EE.

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

Should we delete app/services/ci/create_trigger_request_service.rb?

Why was this MR needed?

This MR is needed for https://gitlab.com/gitlab-org/gitlab-ee/issues/2772 which is Deliverable for %9.5.

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?

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/35282
• Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2604
• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/35584
• Related https://gitlab.com/gitlab-org/gitlab-ee/issues/2772

/cc @ayufan