Add CI_UPSTREAM_PROJECT_ID to pipeline predefined variables

What does this MR do and why?

This MR introduces CI_UPSTREAM_PROJECT_ID as a new pre-pipeline predefined variable for multi-project pipelines.
It lets a downstream pipeline know which upstream project triggered it, without having to manually pass a custom variable.

This MR:

• Adds a downstream_predefined_variables method to Gitlab::Ci::Variables::Downstream::Generator
  • When bridge.triggers_cross_project_pipeline? is true, stores the upstream project ID in CI_UPSTREAM_PROJECT_ID.

References

Related to #377694

How to set up and validate locally

1. In your GDK instance, create two projects (or reuse existing ones):

   • an upstream project
   • a downstream project

2. In the downstream project, create a .gitlab-ci.yml like:

   stages:
     - test
   
   print_upstream_vars:
     stage: test
     script:
       - echo "CI_UPSTREAM_PROJECT_ID=$CI_UPSTREAM_PROJECT_ID"
       - echo "CI_UPSTREAM_PIPELINE_ID=$CI_UPSTREAM_PIPELINE_ID"
       - echo "CI_UPSTREAM_JOB_ID=$CI_UPSTREAM_JOB_ID"

3. In the upstream project, create a .gitlab-ci.yml that triggers the downstream project via a bridge job, for example:

   stages:
     - trigger
   
   trigger_downstream:
     stage: trigger
     trigger:
       project: <your-namespace>/<downstream-project-path>
       strategy: depend

4. Run a pipeline in the upstream project (for example, by pushing a commit or using Run pipeline in the UI).

5. When the downstream pipeline is created and the print_upstream_vars job runs, open the job log.

Expected result

• CI_UPSTREAM_PROJECT_ID is set to the ID of the upstream project.
• CI_UPSTREAM_PIPELINE_ID is set to the ID of the upstream pipeline that triggered the bridge.
• CI_UPSTREAM_JOB_ID is set to the ID of the bridge job in the upstream pipeline.
• All three variables are available as predefined variables in the downstream job’s environment.

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