Improvements to `Project::Infra::Google Cloud`

- Feature flag `incubation_5mp_google_cloud` bound to Project
  This allows for easier testing on gitlab.com where the feature
  flag may be enabled for select projects.

- `service_accounts` is an active route for `google_cloud` menu item
  This expands the appropriate project sidemenu item when creating
  a service account

- Introduce a top level `app.vue` component
  Achieves consistency with a majority of frontend code in the repo
  Where a single route with a single entry point serves multiple templates
  Instead of routing the frontend in `index.js` use a top level component
  to render the appropriate `screen`

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75295