Introduce DeploymentsController with placeholder routes

What does this MR do and why?

While everything is locked up behind a feature flag, we expand the 5mp Google Cloud implementation by introducing the DeploymentsController and two placeholder routes for cloud_run and cloud_storage.

Tests in-place to make sure right access levels. And moved some common methods into the BaseController.

Screenshots or screen recordings

None. Purely backend with placeholder routes.

How to set up and validate locally

1. Configure your GitLab instance for Google OAuth2
2. Enable the incubation_5mp_google_cloud feature flag
3. Navigate to project/-/google_cloud/deployments/cloud_run and project/-/google_cloud/deployments/cloud_run as maintainer or owner to see the placeholder text being rendered.

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.