Part 1 - Pipeline Mini Graph downstream jobs

What does this MR do and why?

Part 1 for #393672 (closed)

NO FUNCTIONALITY CHANGE

• Abstracting downstream ci icons to a new file, downstream_pipeline_dropdown.vue
• Stubbing out the query we will use as well as adding it to fixtures

Part 2 (in dev) will wire in the downstream dropdowns.

Screenshots or screen recordings

No UI change

How to set up and validate locally

1. In a project with CI setup, create a pipeline with downstream

Build > Pipeline editor, enter this for your gitlab-ci.yml and click "Commit changes"

bridge_job:
  stage: build
  trigger:
    include: child-pipeline.yml

bridge_job_2: 
  stage: deploy
  trigger:
    include: child-pipeline.yml

bridge_job_3:
  stage: test
  trigger:
    include: child-pipeline.yml

2. Insure that the downstream pipeline renders in any of the following locations:

• Build > Pipelines - table
• Build > Pipeline editor - pipeline widget
• MR page - pipeline widget, pipelines tab
• Commit page - pipeline widget, pipelines tab