Introduce `Google Cloud` page under `Projects :: Infrastructure`

Related issue: gitlab-org/incubation-engineering/five-minute-production/meta#6 (closed)

What does this MR do and why?

• Introduces a Google Cloud page in the Projects :: Infrastructure section
• This page is hidden behind a feature flag incubation_5mp_google_cloud
• Only project maintainers should have access, policy rule :manage_project_google_cloud
• Mock up for page contents: Deployments and Service Accounts section
• Test, specs & MR hygiene
  • Test new route added projects/-/google_cloud
    • Test that this must be only available to maintainers
  • Sidebar should show Google Cloud for a Project under Infrastructure
  • Fix regressions in existing tests
  • Code style and linters

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag incubation_5mp_google_cloud on your GDK
2. Navigate to any project where you are owner / maintainer
3. You should see the Google Cloud in the sidebar under Infrastructure

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.