UX for simpler GAM and IAM setup

Create designs and prototype to make the proposal in &13197 more concrete and implementable.

README

Whenever we decide to pick this up again, review the thread: #454343 (comment 1962082317)

Links

• 🖼️ Latest wireframes in Figma
• ▶️ Prototypes:
  • Google Artifact Management integration:
    • IAM not set up
    • IAM already set up
  • Google Cloud IAM integration
  • Register runner