[Feature flag] Rollout of `duo_evaluation_ready`

How this feature flag is used

When evaluation runner kicks off an evaluation pipeline which launches a remote GDK instance, it executes wait-for-instance job to make sure the instance is ready for evaluation, example. We're currently checking the HTTP status from GitLab-Rails url, however this technically returns 200 even before the post-deployment script is complete. We enable the duo_evaluation_ready feature flag at the end of the post-deployment script, and change the wait-for-instance job to request to http://gdk.test:3000/api/v4/features endpoint to get the state of the FF. So that the evaluation won't start until the GDK is ready.