Passing variables to CI/CD pipelines in all possible scenarios

We have several ways to trigger a pipeline or to run a single job, and all of them should support passing variables that will have the highest priority (https://docs.gitlab.com/ee/ci/variables/#priority-of-variables).

We might want to pass variables in these cases:

• Run Pipeline button in CI/CD > Pipelines page: https://gitlab.com/gitlab-org/gitlab-ce/issues/44059
• Pipeline Schedules UI: #32568 (closed)
• Pipeline Schedules API: #34518 (closed)
• Manual jobs: #24935 (closed)
• Triggers UI
• Triggers API
• Triggers API when using $CI_JOB_TOKEN: https://gitlab.com/gitlab-org/gitlab-ee/issues/2772
• Pipelines API (create): https://gitlab.com/gitlab-org/gitlab-ce/issues/25045

Related:

• Make trigger variables available from API/Webhook: #28105 (closed)