Support additional variables in the include field

This is a follow up from #219065 (closed) and #276515 (closed) where we implement support for a set of variables

• Project Predefined variables ✅
• Project variables ✅
• Group variables ✅
• Instance variables ✅
• Variables added when a pipeline is created manually via the UI or API 🚫
• Pipeline predefined variables 🚫

This issue is for completing the list of supported variables

Documentation

Based on the discussion below we should document that we will not have all pipeline predefined variables. For example; we won't have CI_PIPELINE_IID and CI_PIPELINE_CREATED_AT because the pipeline doesn't persist yet. (we can add additional variables if we'll discover more)