[keda] Adds keda ClusterTriggerAuthentication for mimir
What
Adds a ClusterTriggerAuthentication object for use with Mimir, and deploys the required secret.
Why
This is required to enable Keda to authenticate with Mimir. A cluster level authentication object prevents the need to deploy a TriggerAuthentication CRD per namespace/service using ScaledObjects.
Related issue: gitlab-com/gl-infra/scalability#3451 (comment 1905607197)
Edited by Nick Duff