Show linked pipelines mini graph in the pipeline editor

What does this MR do and why?

For #337514 (closed)

This adds the upstream/downstream pipeline mini graphs in the pipeline editor.

The feature is under the pipeline_editor_mini_graph feature flag. Since we're not enabling the FF yet, no changelog trailers are added.

Screenshots or screen recordings

State	Screenshot
Desktop View
Mobile View
With Error

Note: Fetching the linked pipelines occurs in a separate call from fetching the data for the main mini pipeline graph. So it's possible to load the mini pipeline graph without loading the linked pipelines.

How to set up and validate locally

1. Enable the pipeline_editor_mini_graph feature flag. (rails c then Feature.enable(:pipeline_editor_mini_graph))

2. Go to the pipeline editor (CI/CD > Editor)

3. If your project doesn't have downstream pipelines, you can add the following to your .gitlab-ci.yml file.

   stages:
     - prepare
   
   prepare_job:
     stage: prepare
     script:
       - echo "running prepare_job"
   
   child_pipeline:
     stage: prepare
     trigger:
       include:
         - project: "path/to/project"
           file: ".gitlab-ci.yml"

   You can replace /path/to/project with some other existing project e.g. gitlab-org/gitlab-shell or create a separate project with the a simple .gitlab-ci.yml content:

   stages:
     - prepare
   prepare_job:
     stage: prepare
     script:
       - echo "running prepare_job"

4. Verify that the mini pipeline graph shows up in the pipeline status.

MR acceptance checklist

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