Refactor Runner services and GraphQL queries to depend on the new IAM integration

The GoogleCloudPlatform::ArtifactRegistry was introduced in !141127 (merged). Back then we decided to use this integration not only to host the GAR specific settings (project ID, repository location and repository name), but also the identity settings (project, pool and provider ID), which is being used by the runner for accessing Google Compute.

We have later decided in #439036 (closed) to create a new "parent" integration to host the WIF settings. Therefore, once this integration is available (https://gitlab.com/gitlab-org/gitlab/-/issues/439200), we should update the runner code to source identity settings from the new IAM integration;