Cells: Runner blueprint

What does this MR do and why?

Adds a blueprint for the Runner service relative to a Cell.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• GRIT feedback
• Runner feedback
• Delivery feedback
• Dedicated feedback
• Update https://docs.gitlab.com/ee/architecture/blueprints/cells/infrastructure

How to set up and validate locally

Review https://gitlab.com/gitlab-org/gitlab/-/blob/fd9e0e410f2cd3a5f9ece098bd9c73fbad40400d/doc/architecture/blueprints/cells/infrastructure/runner.md.