WIP: Resolve "Projects::PipelinesController#index.json is slow due to SQL"

What does this MR do?

Eliminating all the N+1 queries when using PipelineSerializer

Are there points in the code the reviewer needs to double check?

Hopefully the behaviour would stay the same.

Why was this MR needed?

Speed up Projects::PipelinesController#index.json so that we could use this to update pipeline table in realtime (i.e. requesting this so much more frequently)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug

What are the relevant issue numbers?

Closes #32766 (moved)