Disable Open in Google Cloud button unless setup is valid

Context

Part of Google Artifact Management Integration for Cont... (&13124 - closed).

Problem

Right now we always display the Open in Google Cloud button on the artifacts list view, even if there is a problem connecting to GAR:

This can lead to confusion as users may have the required permissions to see the AR repo on the GCP side but then see an error on the GitLab side.

Proposal

Make the button inactive if unable to list images. Ideally, with GAR integrations: Implement configuration test ... (gitlab#438560 - closed) we should be able to guard against most misconfiguration errors, but a situation like the one described may still be possible after that.